Preheat oven to 350F. Lightly grease one 6-inch round cake pan then line the bottom with parchment paper.

In a medium bowl, sift together the flour and cornstarch then whisk in cocoa powder, baking soda, and salt.

In a large mixing bowl, beat together the sugar and oil until moistened and smooth, about 1 minute.

Beat in the egg, food colouring, vanilla, and vinegar.

Add the flour mixture alternately with the buttermilk, adding the flour in 3 additions and the milk in 2 additions (begin and end with dry ingredients).

Transfer the batter to the pan. Bake 20-23 minutes or until a toothpick inserted in the middle comes out with a few crumbs (no wet batter). Cool for about 15 minutes, run a knife along the edge of the pan to loosen and invert onto a cooling rack. Cool right-side-up.

Decorate with buttercream and sprinkles of your choice.