Cream the butter and sugar on medium speed for about 2 minutes, until pale and fluffy.
115 g unsalted butter, 300 g sugar
Add the eggs one at a time, beating well after each addition.
2 eggs
In a separate bowl, whisk the red food coloring into the buttermilk.
20 g red gel food coloring, 240 ml buttermilk
With the mixer on low speed, add the flour mixture and buttermilk alternately, beginning and ending with the flour mixture. Mix well after each addition.
Add the vanilla and mix just until no streaks of flour remain.
1 teaspoon vanilla extract
Remove the bowl from the mixer. Stir together the baking soda and vinegar, then quickly fold it into the batter as soon as it foams.
1 teaspoon baking soda, 1 teaspoon vinegar
Divide the batter evenly between the pans, then bake for 24 to 30 minutes, until the cakes pull away from the sides and a toothpick inserted in the center comes out clean.
Let the cakes cool on a wire rack for about 15 minutes. Unmold, remove the parchment, and let cool completely before frosting.
Ermine Frosting
In a saucepan, whisk together the all-purpose flour and milk.
5 tablespoons all-purpose flour, 240 ml milk
Cook over medium heat, stirring constantly, for 8 to 10 minutes, until very thick, like pastry cream.
Remove from the heat, press plastic wrap directly onto the surface, let cool to room temperature, then refrigerate for 1 hour.
Cream the sugar, butter, and vanilla on medium-high speed for 3 to 5 minutes, until pale and fluffy.
200 g sugar, 225 g unsalted butter, 1 teaspoon vanilla extract
Add the cooled flour mixture and beat for 3 to 5 minutes, until light and fluffy, like whipped cream.
Fill, frost, and decorate the cake.
Notes
Ermine frosting—made by cooking flour and milk—is the traditional frosting for red velvet cake. It's lighter and less sweet than cream cheese frosting. Natural cocoa, buttermilk, and vinegar give the cake its signature color and a subtly tangy flavor. For the best texture and cleanest slices, chill the cake for at least 2 hours, then let it stand at room temperature for 10 minutes before serving.