"A buttery, crisp Pop-Tart® crust and a moist cinnamon brown sugar cake. Great for all ages and a fun party idea. I made this cake for my family and they all loved it (and some of them don't even like Pop-Tarts®!)."
Preheat oven to 325 degrees F (165 degrees C). Spray an 8-inch square pan.
Crumble 4 toaster pastries into a bowl and mix in melted butter; press into the prepared pan creating a crust.
Combine 1/4 cup brown sugar, 1 tablespoon flour, 1 teaspoon cinnamon, and 1 teaspoon milk in a bowl; spread over crust.
Beat 1/2 cup brown sugar, softened butter, and white sugar together in a bowl; add egg yolk and vanilla extract and mix well. Combine 1 cup flour, baking powder, and salt in a separate bowl. Whisk 3/4 cup milk and vinegar together in a separate bowl.
Mix flour mixture, alternating with milk-vinegar mixture, into butter mixture until batter is well mixed. Fold egg whites into batter. Crumble 1 toaster pastry over batter and fold. Pour batter over the crust.
Bake in the preheated oven until a toothpick inserted in the center comes out clean, 45 to 50 minutes. Cool cake completely, at least 1 hour.
Spread frosting onto cake and crumble 1 toaster pastry over frosting.