Easy Churro Cake Recipe with Cinnamon Sugar filling

0 Preheat oven to 350°F (175°C).
1 Line the bottom of an 8×8-inch square baking pan with parchment paper, leaving overhang on two sides. Grease and lightly flour the sides.
2 In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
3 In a large bowl using an electric mixer, beat the butter and granulated sugar in medium-high speed for about 2 minutes, until light and fluffy.
4 Add the eggs, one at a time, followed by the egg white and vanilla extract. Mix until fully combined.
5 Gradually add the dry ingredients to the wet ingredients, mixing just until incorporated. Do not overmix.
6 In a small bowl, mix together the brown sugar, cinnamon, and flour for the filling.
7 Spread half of the cake batter evenly into the prepared pan.
8 Sprinkle the entire cinnamon sugar mixture evenly over the batter, reaching the edges.
9 Dollop the remaining batter on top and gently spread to cover. It’s okay if it gets a bit messy.
10 Bake for 25-30 minutes, or until a toothpick inserted in the center comes out mostly clean (moist crumbs from the cinnamon layer are normal).
14 In a small bowl, mix the cinnamon and sugar for the topping.
11 Let the cake cool in the pan for 10–15 minutes, then carefully remove it and transfer to a wire rack. 
12 While the cake is still warm, brush the top with melted butter and sprinkle evenly with the cinnamon sugar topping. Let cool completely.