Cream together butter and sugar with an electric mixer on medium speed until light and fluffy. Add eggs, one at a time, mixing after each addition.

Mix cocoa powder and food coloring in a small bowl to make a paste; add to butter mixture and mix until well combined.

Mix salt, vanilla, and buttermilk in a small bowl; add to butter mixture alternately with cake flour until well combined.

Mix baking soda and vinegar in a small bowl, then fold into the batter by hand until well combined

Pour batter into two 9-inch greased and floured cake pans, then bake at 350 degrees for 30 minutes or until cooked. Cool completely, then frost with Cooked Buttercream Frosting. The cake does not need to be refrigerated.

Add flour to a small saucepan, then whisk in milk. Heat over medium-high heat until boiling. Cook and stir until thickened (about the consistency of your cake batter). Remove from heat and cool completely.

Cream butter and sugar with an electric mixer until fluffy and smooth (about 3-4 minutes). Add vanilla and cooked flour mixture and continue mixing until creamy, well combined, and spreadable. Don’t worry if it doesn’t seem to be coming together – it will. Just keep mixing!