For the cupcakes
3 tablespoons Dutch-process cocoa powder
1/4 cup hot water
1 1/4 cups unbleached all-purpose flour
1/2 teaspoon baking powder
1/2 teaspoon baking soda
1/4 teaspoon salt
2 large eggs, at room temperature
3/4 cup granulated sugar
1/2 cup buttermilk, at room temperature
1/2 teaspoon vanilla extract
1/4 cup unsalted butter, melted and cooled to room temperature
For the frosting
6 ounces bittersweet chocolate
1 cup unsalted butter, at room temperature
2 cups confectioners’ sugar
1. Preheat the oven to 350°F (175°C). Line 12 standard muffin cups with paper liners.
2. In a small bowl, stir the cocoa powder into the hot water until it dissolves; set aside. Sift the flour, baking powder, baking soda, and salt together into a bowl. Using the finest rasps of a handheld grater, grate the zest from the orange into the bowl. Set aside.
3. In a large bowl, whisk together the eggs and granulated sugar until well combined. Whisk in the buttermilk and vanilla, then the dissolved cocoa. Whisk in the melted butter, then the dry ingredients.
4. Using a tablespoon, divide the batter among the muffin cups filling each about half full. Bake until the cupcakes are puffed and a skewer inserted into the center of one comes out clean 15-20 minutes. Let cool completely on a wire rack. Remove the cupcakes from the pan.
5. To make the frosting, melt the chocolate and let cool to room temperature. Meanwhile, using a stand mixer, beat the butter and confectioners’ sugar with the paddle on medium speed until creamy and smooth, about 3 minutes. Beat in the melted chocolate until combined. Fill a pastry bag fitted with a 1/2-inch star tip with the frosting and pipe a spiral on top of each cupcake.
6. Refrigerate the cupcakes until 30 minutes before serving to set the frosting.