Prep the pan: Heat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment, leaving overhang for easy lifting.
Lightly grease.
Make the cinnamon sugar: In a small bowl, combine 2 tablespoons sugar with 1 teaspoon cinnamon. Set aside for topping.
Whisk the wet ingredients: In a large bowl, whisk melted butter, brown sugar, and 1/4 cup granulated sugar until glossy and combined, about 30 seconds. Whisk in eggs and vanilla until smooth.
Combine dry ingredients: In a separate bowl, whisk flour, baking powder, salt, and the remaining 1 teaspoon cinnamon.
Mix the batter: Add dry ingredients to wet and stir with a spatula just until no dry streaks remain.
The batter will be thick.
Pan and top: Spread batter evenly in the pan. Sprinkle the cinnamon sugar topping over the surface for a crinkly, sweet crust.
Bake: Bake 23–28 minutes, until the edges are set and a toothpick inserted near the center comes out with a few moist crumbs. Do not overbake.
Cool and slice: Cool in the pan for at least 20–30 minutes.
Lift out using the parchment and cut into 16 squares.