- Preheat oven to 350°. Grease and lightly flour three 9- by 2-inch round cake pans, then line the bottoms with waxed paper, or put 30 cupcake papers in muffin pans.
- To make the cake: Sift the flour into a small bowl or onto waxed paper. In a large bowl, on the medium speed of an electric mixer, cream the butter and sugar until very light and fluffy, about 5 minutes. Add the eggs, one at a time, beating well and scraping the bowl after each addition. Add the red food coloring, cocoa, espresso powder, vanilla and salt and beat until fully incorporated.
- Add the buttermilk to the batter in three parts, alternating with the flour. With each addition, beat until the ingredients are incorporated, but do not overbeat. In a small bowl, stir together the cider vinegar and baking soda. Add to the batter and mix well. Using a rubber spatula, scrape down the batter in the bowl, making sure the ingredients are well blended and the batter is smooth.
- Divide the batter among the prepared pans. Bake for 30 to 40 minutes, or until a cake tester inserted in the center of the cake comes out clean. (Cupcakes will take about 22 minutes.) Cool in pans for 1 hour. Remove from the pans and cool completely on a wire rack. While the cake is cooling, make the frosting.
- When the cake has cooled, spread the frosting between the layers, then ice the top and sides of the cake.
- To make the frosting: In a mediumsize saucepan, off the heat, whisk the flour into the cold milk until smooth. Place over medium heat and, stirring constantly, cook until the mixture becomes very thick and begins to bubble, 10 to 15 minutes. Remove from heat. Cover with waxed paper placed directly on the surface and cool to room temperature, about 30 minutes.
- In a large bowl, on the medium high speed of an electric mixer, beat the butter for 3 minutes, until smooth and creamy. Gradually add the sugar, beating continuously for 3 minutes until fluffy. Add the vanilla and beat well.
- Add the cooled milk mixture, and continue to beat on the medium high speed for 5 minutes, until very smooth and noticeably whiter in color. Cover and refrigerate for at least 15 minutes. Use immediately for best spreading. If you must keep it refrigerated longer, let it come almost to room temperature before using. Stir it gently before using.
- This makes a 3-layer cake, or about 30 regular-size cupcakes. It is very easy to cut the recipe and frosting recipes in half — you will get around 15 cupcakes.