3 1/2 cups cake flour (not self-rising)
3/4 cup unsalted butter, softened
2 1/2 cups sugar
3 large aggs, room temperature
6 tbsp red food colouring
3 tbsp unsweetened cocoa
1 1/2 tsp vanilla extract
1 1/2 tsp salt
1 1/2 cups buttermilk
1 1/2 tsp cider vinegar
1 1/2 tsp baking soda
6 tbsp all-purpose flour, for the vanilla frosting
2 cups milk, for the vanilla frosting
2 cups unsalted butter, softened, for the vanilla frosting
2 cups sugar, for the vanilla frosting
2 tsp vanilla extract, for the vanilla frosting
Preheat oven to 350F. Grease and lightly flour 2 cupcake pans (24 cupcakes).
To make the batter: In a small bowl, sift the cake flour and set aside. In a large bowl, on the medium speed of an electric mixer, cream the butter and sugar until very light and fluffy, about 5 mins.
Add the eggs, one at a time, beating well after each addition. In a small bowl, whisk together the red food coloring, cocoa, and vanilla. Add to the batter and beat well.
In a measuring cup, stir the salt into the buttermilk. Add 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. Arrange the oven racks in the upper and lower thirds of the oven and bake the cupcakes, switching positions of the pans halfway through baking, until a tester comes out clean, about 20 minutes.
Cool the cupcakes in the pan 10 mins, then remove from the pan and cool completely on a rack before icing. To ice, mound about 1/4 cup of frosting on top of each cupcake and use an icing spatula to make a swirl on top. Decorate with red sanding sugar.
To make the vanilla frosting: In a medium-size saucepan, whisk the flour into the milk until smooth. Place over medium heat and, stirring constantly, cook until the mixture becomes very thick and begins to bubble, 10-15 mins.
Cover with waxed paper placed directly on the surface and cool to room temperature, about 30 mins.
In a large bowl, on the medium high speed of an electric mixer, beat the butter for 3 mins, until smooth and creamy. Gradually add the sugar, beating continuously for 3 mins 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 15 mins (no less and no longer—set a timer!). Use immediately.