Preheat an oven to 350°F. Lightly coat a 9 x 13 x 2-inches baking pan with cooking oil, then line it with parchment paper.

Make the cake batter by whisking together flour, cornstarch, sugar, baking powder, and baking soda in a stand mixer.

Mix in butter and vanilla extract until the mixture resembles damp sand.

Whisk together buttermilk and eggs and then drizzle the mixture into the butter and dry ingredients.

Divide the batter into 6 bowls and color each with food coloring.

Alternate spoonfuls of colored batter into the prepared pan and lightly swirl.

Bake for 30 to 35 minutes until a toothpick comes out clean. Cool completely.

For the frosting, cream together softened cream cheese and butter, then incorporate vanilla and sifted powdered sugar.

Frost the cake and sprinkle with rainbow sprinkles before serving.