Preheat oven to 350°F. Grease two 8- or 9-inch round cake pans and set aside.
In a medium bowl, whisk the cake mix and cocoa powder together.
3 tablespoons unsweetened cocoa powder, 18 ounces yellow cake mix
In a second larger mixing bowl, using a hand mixer, beat the butter until light and fluffy. Add the eggs one at a time, beating after each addition.
½ cup unsalted butter, 2 large eggs
Stir in the food coloring, vinegar and vanilla, mixing well.
2 tablespoons red food coloring, ½ tablespoon white vinegar, 1 tablespoon pure vanilla extract
Mix in the cake mix and buttermilk in alternating batches, beginning and ending with the cake mix mixture.
1 cup buttermilk
Divide the batter between the prepared cake pans (about 506 grams per pan).
Bake for 20-30 minutes, or until a toothpick inserted in the center comes out clean.
Cool the layers for 10 minutes before removing them from the pans to wire racks to cool completely, about 2 hours.