Preheat oven to 350 degrees. Butter three 9-inch cake rounds. Dust with flour and tap out the excess.
Mix together flour, sugar, cornstarch, cocoa, baking soda, baking powder, and salt in a stand mixer using a low speed until combined.
Add eggs, buttermilk, warm water, oil, vanilla, vinegar, and food coloring. Beat on a medium speed until smooth. This should take just a couple of minutes.
Divide batter among the three prepared pans.
Bake for 30-35 minutes until the cake meets the toothpick test (stick a toothpick in and it comes out clean).
Cool on wire racks for 15 minutes and then turn out the cakes onto the racks and allow to cool completely before frosting.
Make the frosting. In a large bowl, beat together butter and cream cheese until fluffy. Use a hand mixer or stand mixer for best results. Add in vanilla extract. Beat until combined. Beat in powdered sugar, 1 cup at a time until frosting is smooth.
Assemble and frost the completely cooled cake.
Notes
For a brighter red color, reduce the cocoa powder to 1/4 cup.