Preheat the oven to 350℉. Line the bottom of a 9”x13” pan with parchment paper. Set aside.
Sift together the cake flour, cornstarch, baking powder and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment or in a large mixing bowl using an electric mixer, cream the butter and sugar together until light and fluffy.
Add the egg, vanilla and coconut extracts and mix until well blended. Add the sour cream and beat until fully combined.
Slowly add the dry ingredients to the wet ingredients, mix until just combined. Do not overmix.
Drop the batter by large spoonfuls into the prepared dish. The batter will be very thick. Spread batter evenly for uniform baking.
Bake for 18-20 minutes or until the edges are slightly browned. Don’t overbake. Cool completely before frosting.