From The Velvet Truth
The classic Red Velvet, with a cooked frosting that mimics whipped cream, in place of cream-cheese frosting.
1/2 cup butter
1 1/2 cups sugar
2 eggs
4 teaspoons red food coloring
3 tablespoons cocoa (heaping)
1 cup buttermilk
2 1/4 cups cake flour
1 teaspoon salt
1 teaspoon vanilla
1 teaspoon baking soda
1 teaspoon vinegar
Preheat oven to 350 degrees. Butter and flour two, 8-inch round cake pans.
In large mixing bowl, cream butter, sugar and eggs.
In a small bowl, make a paste of the food coloring and cocoa. Add to creamed mixture and blend well.
In separate bowl, mix together flour and salt. Add to creamed mixture, alternating with buttermilk. Add vanilla.
In a small bowl, add baking soda to vinegar. Blend into the batter.
Pour batter into cake pans and bake for 25-30 minutes. Cool completely in pans.
FROSTING:
3 tablespoons flour
1 cup milk
1 cup sugar
1 teaspoon vanilla
1 cup butter
In small, heavy bottomed saucepan, whisk milk into flour slowly, avoiding lumps. Cook over medium high heat until very thick, stirring constantly. Set aside to cool completely. (Can be made ahead and refrigerated.)
In large bowl, cream sugar, vanilla and butter. Add cooked milk-flour mixture. Beat on high speed, until very fluffy.
Frosting will resemble whipped cream.
Three Points Kitchen 
Make sure not to use “Dutch” (or alkali) processed cocoa if you want to get the reaction that turns it a reddish color. The alkali processing changes the pH & won’t get you the red color. Look for “natural” cocoa that doesn’t use that process, for red velvet cakes (that aren’t using red dye, but relying on the chemical reaction between the acidic ingredients and components in the cocoa.)