-
Preheat oven to 350° F. Butter the bottom of three 8-inch round cake pans, line with parchment rounds, and dust with flour, tapping out the excess.
-
In a stand mixer fitted with the paddle attachment, beat the butter, sugars, red velvet flavor, and red gel paste on medium-high speed until lighter in color and slightly increased in volume, about 5 minutes. Lower the speed to medium and add the eggs, one at a time, mixing until each is fully incorporated before adding the next.
-
Sift the flour and cocoa powder into a medium bowl. Add the salt to the dry ingredients after sifting, and whisk dry ingredients. Alternate dry ingredients and buttermilk into creamed mixture, beginning and ending with dry ingredients. Mix until just incorporated, or finish by hand gently. Fold in mayonnaise.
-
In a small cup or bowl, combine the vinegar and baking soda until it fizzes, then quickly add the batter and whisk until blended.
-
Divide the batter evenly between the prepared pans. If possible, weigh the batter in each cake pan for 460 g each (excluding the pans--you will want to tare the scale each time.) This ensures even layers. Smooth with small offset palette knife, and bake on top of a baking sheet (two pans on sheet for first round, then the final pan) until a toothpick inserted in the center comes out clean, about 25 minutes. Try not to over-bake.
-
Let pans cool on wire rack for 20 minutes, then invert cakes onto racks, gently, peeling away parchment rounds. Let cool completely.