Servings: Makes 36 cupcakes
1/4 cup dark unsweetened cocoa powder
2 Tbsp. red no-taste gel food coloring
1/4 cup boiling water
6 Tbsp. unsalted butter , cut into small pieces and softened
2 Tbsp. room temperature vegetable shortening
1 2/3 cups sugar
3 large eggs
1 cup buttermilk
1 tsp. pure vanilla extract
2 1/2 cups cake flour
1 tsp. salt
1 Tbsp. cider vinegar
1 tsp. baking soda
1 1/2 cups sugar
1/4 cup all-purpose flour
1 1/2 cups milk
1/4 cup heavy cream
3 sticks unsalted butter , soft but cool, cut into small pieces
1 tsp. pure vanilla extract
2 tsp. cinnamon
Cinnamon imperials candies (optional)
Red white-chocolate hearts (optional)
To make cupcakes: Preheat oven to 325°. Line 36 (2 1/2-inch) muffin pan cups with paper liners. In a medium bowl, whisk together cocoa powder, food coloring, and boiling water. Set aside to cool. In the bowl of an electric mixer fitted with a paddle attachment, cream butter and shortening until smooth. Scrape down bowl and add sugar. Beat until mixture is light and fluffy, about 5 minutes. Add eggs, one at a time, beating well after each addition.
Stir buttermilk and vanilla into the cooled cocoa mixture. Sift cake flour and salt together into another bowl. With the mixer on low, alternate adding the flour mixture (in 3 parts) and the cocoa mixture (in 2 parts) to the egg mixture. Beat until incorporated. In a small bowl, combine vinegar and baking soda and stir until baking soda dissolves; the mixture will fizz. Add to batter and stir until just combined.
Fill each cupcake liner about 2/3 full with batter. Bake 20 to 25 minutes, or until a toothpick inserted in the center comes out clean. Remove from oven, transfer to a wire rack, and let cool for 10 minutes. Remove cupcakes from pan, and let cool completely.
To make buttercream frosting: In a heavy-bottomed saucepan, whisk the sugar and flour together. Add the milk and cream and cook over medium heat, whisking occasionally, until the mixture boils and has thickened, about 12 minutes. Transfer the mixture to the bowl of an electric mixer fitted with the paddle attachment. Beat on medium-high speed until cool, 5 minutes. Reduce the speed to low and add the butter; beat until thoroughly incorporated. Increase speed to medium-high and beat until frosting is light and fluffy, 3 minutes. Add the vanilla and cinnamon and continue mixing until combined. If the frosting is too soft, transfer the bowl to the refrigerator to chill slightly, then beat again until it is the proper consistency. If the frosting is too firm, place the bowl over a pot of simmering water and beat with a wooden spoon until it is the proper consistency.
Frost each cupcake with buttercream. Garnish with cinnamon imperials and white-chocolate hearts, if desired