Oat Flour Blueberry Crumble Squares – Simple, Cozy, and Naturally Sweet

These crumble squares bring together juicy blueberries and a buttery oat crust for a treat that tastes like summer, any time of year. The base is tender, the topping is crunchy, and the filling cooks down into a jammy layer that holds everything together. They’re easy enough for a weekday bake but special enough to share.

You can make them with fresh or frozen berries, and they slice cleanly once cooled. Keep a pan on the counter for snacks, breakfast, or an after-dinner dessert.

Print Recipe

Oat Flour Blueberry Crumble Squares – Simple, Cozy, and Naturally Sweet

Prep Time15 minutes mins

Cook Time35 minutes mins

Total Time50 minutes mins

Servings: 9 servings

Ingredients

• For the crust and crumble:
• 2 cups oat flour (store-bought or finely ground rolled oats)
• 1 cup old-fashioned rolled oats
• 1/2 cup light brown sugar, packed
• 1/4 cup granulated sugar
• 1/2 teaspoon fine sea salt
• 1/2 teaspoon ground cinnamon (optional)
• 1/2 teaspoon baking powder
• 10 tablespoons unsalted butter, melted and slightly cooled
• 1 teaspoon vanilla extract
• For the blueberry filling:
• 3 cups blueberries (fresh or frozen; no need to thaw)
• 2 tablespoons lemon juice
• 1 teaspoon lemon zest
• 1/3 cup granulated sugar (adjust to berry sweetness)
• 2 tablespoons cornstarch
• 1 teaspoon vanilla extract
• Pinch of salt

Instructions

• Preheat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang for easy lifting.Lightly grease the sides.
• Make the crust and crumble: In a large bowl, whisk oat flour, rolled oats, brown sugar, granulated sugar, salt, cinnamon, and baking powder. Pour in melted butter and vanilla. Stir until the mixture looks like damp sand and clumps when pressed.
• Form the base: Transfer about 2/3 of the mixture to the pan.Press it firmly and evenly into a compact crust. Use the bottom of a measuring cup to pack it well.
• Mix the filling: In another bowl, toss blueberries with lemon juice, zest, sugar, cornstarch, vanilla, and a pinch of salt until evenly coated.
• Assemble: Spread the blueberry mixture over the crust in an even layer. Sprinkle the remaining crumble over the top, squeezing some of it into larger clumps for texture.
• Bake for 40–50 minutes, until the top is golden and the filling is bubbling around the edges.If using frozen berries, add 5 minutes.
• Cool completely in the pan on a rack—at least 2 hours. This sets the filling so the squares slice cleanly.
• Lift out using the parchment and cut into 12–16 squares. Serve as is or with a spoonful of yogurt or ice cream.

Why This Recipe Works

• Oat flour keeps it tender: Oat flour brings a soft, delicate crumb while still feeling hearty.
• Two-in-one dough: One mixture doubles as the crust and crumble topping, saving time and dishes.
• Blueberries do the heavy lifting: A little lemon and cornstarch turn berries into a thick, glossy filling.
• Balanced sweetness: Just enough sugar to highlight the berries without overpowering them.

Ingredients

• For the crust and crumble:
• 2 cups oat flour (store-bought or finely ground rolled oats)
• 1 cup old-fashioned rolled oats
• 1/2 cup light brown sugar, packed
• 1/4 cup granulated sugar
• 1/2 teaspoon fine sea salt
• 1/2 teaspoon ground cinnamon (optional)
• 1/2 teaspoon baking powder
• 10 tablespoons unsalted butter, melted and slightly cooled
• 1 teaspoon vanilla extract
• For the blueberry filling:
• 3 cups blueberries (fresh or frozen; no need to thaw)
• 2 tablespoons lemon juice
• 1 teaspoon lemon zest
• 1/3 cup granulated sugar (adjust to berry sweetness)
• 2 tablespoons cornstarch
• 1 teaspoon vanilla extract
• Pinch of salt

Instructions

1. Preheat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving overhang for easy lifting.

   Lightly grease the sides.

2. Make the crust and crumble: In a large bowl, whisk oat flour, rolled oats, brown sugar, granulated sugar, salt, cinnamon, and baking powder. Pour in melted butter and vanilla. Stir until the mixture looks like damp sand and clumps when pressed.
3. Form the base: Transfer about 2/3 of the mixture to the pan.

   Press it firmly and evenly into a compact crust. Use the bottom of a measuring cup to pack it well.

4. Mix the filling: In another bowl, toss blueberries with lemon juice, zest, sugar, cornstarch, vanilla, and a pinch of salt until evenly coated.
5. Assemble: Spread the blueberry mixture over the crust in an even layer. Sprinkle the remaining crumble over the top, squeezing some of it into larger clumps for texture.
6. Bake for 40–50 minutes, until the top is golden and the filling is bubbling around the edges.

   If using frozen berries, add 5 minutes.

7. Cool completely in the pan on a rack—at least 2 hours. This sets the filling so the squares slice cleanly.
8. Lift out using the parchment and cut into 12–16 squares. Serve as is or with a spoonful of yogurt or ice cream.

Keeping It Fresh

• Room temp: Store in an airtight container for up to 2 days.
• Refrigerator: Keeps well for 4–5 days.

  The crust stays firmer when chilled.

• Freezer: Freeze individually wrapped squares in a zip-top bag for up to 2 months. Thaw in the fridge or warm briefly in a low oven.

Why This is Good for You

• Whole grains: Oats offer fiber and a steady release of energy.
• Antioxidants: Blueberries are rich in anthocyanins, which support overall wellness.
• Manageable sweetness: The recipe uses moderate sugar and plenty of fruit flavor.
• Gluten-friendly option: With certified gluten-free oats, this can be suitable for gluten-free baking.

Common Mistakes to Avoid

• Skipping the cool-down: Cutting too soon leads to messy, runny squares.
• Underpacking the crust: A loosely packed base can crumble apart. Press it firmly.
• Not enough thickener: Cornstarch helps set the filling.

  Don’t reduce it unless you enjoy a looser square.

• Overbaking: Aim for golden topping and bubbling edges. Burnt oats taste bitter.
• Using quick oats: They can make the topping soggy. Stick with old-fashioned rolled oats.

Alternatives

• Butter swaps: Use coconut oil (refined for neutral flavor).

  Add a pinch more salt if needed.

• Sugar options: Replace granulated sugar with coconut sugar or maple sugar for a deeper flavor.
• Berry variations: Try raspberries, blackberries, or a mixed-berry blend. Adjust sugar to taste.
• Citrus twist: Orange zest works beautifully with blueberries for a softer, sweeter aroma.
• Nutty add-ins: Stir 1/2 cup chopped pecans or almonds into the crumble for extra crunch.

FAQ

Can I use all-purpose flour instead of oat flour?

Yes. Swap the oat flour with the same amount of all-purpose flour.

The texture will be a bit less hearty but still delicious.

Do frozen blueberries make it watery?

Not if you use cornstarch and bake until it bubbles. Add a few extra minutes to the bake time and don’t thaw the berries first.

How do I make this gluten-free?

Use certified gluten-free oat flour and oats. All other ingredients are typically gluten-free, but check labels to be safe.

Can I cut the sugar?

You can reduce the sugar in the crust by 2–3 tablespoons and in the filling by 1–2 tablespoons.

Keep some sugar for structure and flavor balance.

How do I know it’s done?

Look for a golden-brown top and visible bubbling around the edges. That means the cornstarch has activated and thickened the filling.

What if I don’t have cornstarch?

Use 3 tablespoons of tapioca starch or 4 tablespoons of flour. The texture will vary slightly but still set well.

Final Thoughts

These Oat Flour Blueberry Crumble Squares are simple, satisfying, and adaptable.

The oat base gives a cozy, rustic feel, while the blueberry layer brings bright, jammy flavor. Bake a pan on Sunday, and you’ll have snacks and sweet bites ready all week. They’re the kind of treat that disappears fast—so you might want to make a double batch.