Preheat your oven to 350°F (175°C). Grease and line an 8-inch square pan with parchment, leaving a slight overhang for easy lifting.
In a medium bowl, whisk 1 1/2 cups flour, 1 teaspoon baking powder, 1/2 teaspoon salt, and 1 teaspoon cinnamon.
Set aside.
In a large bowl, whisk 1/2 cup granulated sugar and 1/2 cup packed light brown sugar with 1/2 cup melted unsalted butter until glossy and combined.
Whisk in 2 large eggs, one at a time, then add 1 teaspoon vanilla extract.
Stir in half the dry ingredients, then 2/3 cup buttermilk, then the remaining dry mix. Mix just until smooth; don’t overmix.
Optional swirl: In a small bowl, mix 1/4 cup brown sugar with 1 teaspoon cinnamon. Pour half the batter into the pan, sprinkle the swirl mixture, then top with remaining batter and gently spread.
Make the topping by mixing 1/4 cup granulated sugar with 1 1/2 teaspoons cinnamon.
Sprinkle evenly over the batter. For extra churro flair, dot the top with 1–2 tablespoons very small pieces of cold butter.
Bake 25–32 minutes, until the center springs back and a toothpick comes out clean or with a few moist crumbs.
Cool in the pan for 15 minutes, then lift out using the parchment to finish cooling on a rack. Slice into squares and serve slightly warm for peak churro vibes.