Preheat the oven to 350 degrees F. Grease or line 2 6-inch cake pans with parchment paper.
In a large bowl, whisk together the milk and vinegar and let it stand for about 5 minutes to make ‘buttermilk’.
Add the sugar, oil, and vanilla extract, and whisk together until the mixture becomes frothy. Sift together the flour, cornstarch, baking soda, baking powder, and salt.
Add the dry ingredients to the wet ingredients and whisk together until the flour mixture disappears.
Divide the batter evenly between the pans and bake for 25 to 30 minutes, or until a toothpick inserted into the center of the cake comes out clean.
Cool the pans on wire racks for about 15 minutes. Then turn the cakes out of the pans onto the wire racks and cool completely before frosting.
Mix in the vanilla and powdered sugar a little at a time until it all comes together and looks like frosting.
Using and electric hand mixer, mix the butter until it softens and becomes light and fluffy; about a minute.
To assemble the birthday cake: Place one layer of the cake on a plate and spread about ½ cup of frosting on top evenly over the first layer.
Place the second layer on top of the first and do the same with the frosting and put a large dollop of frosting on the top of the cake.
Work your way around the sides until the entire cake is covered with an even layer of frosting and top with sprinkles.