Preheat oven to 350°F. Prepare 6-inch cake pan by lightly greasing and lining bottom with parchment paper.
In a medium bowl, sift together flour, cornstarch, baking powder, salt and baking soda. Set Aside.
In a large bowl, beat together butter and sugar until fluffy. Beat in egg and vanilla.
Alternate adding the flour mixture and buttermilk, starting and ending with the flour mixture. Beat until all ingredients are incorporated and batter is smooth.
Spread batter evenly into prepared pan. Bake 20-23 minutes or until a cake tester comes out clean. Cool for 15 minutes in pan, then transfer to cool completely on a cooling rack.
To make frosting, beat together butter and cream cheese until fluffy. Add powdered sugar and cocoa powder, beating until smooth. Scrape down sides of bowl and add vanilla. Beat until fluffy and lightened in color.
Smooth icing over top of cooled cake. Add sprinkles if desired and enjoy!