For the cake:
1¾ cups plus 2 tablespoons cake flour
1 cup Dutch-process cocoa powder
1½ tsp. baking powder
¾ tsp. baking soda
½ tsp. salt
6 large eggs at room temperature, separated
1 cup canola oil
1¾ cups plus 2 tablespoons sugar, divided
6 tablespoons water
For the filling and frosting swirl:
2 cups marshmallow creme
½ cup (1 stick) unsalted butter, at room temperature
¾ cup confectioners’ sugar, sifted
Pinch of salt
½ tsp. cream or milk
For the chocolate glaze:
5 oz. bittersweet chocolate, finely chopped
½ cup plus 2 tablespoons heavy cream
To make the cupcakes, preheat the oven to 350˚ F. Line cupcake pans with paper liners. In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. In a large bowl, combine the egg yolks, canola oil, 1½ cups of the sugar and the water. Whisk to blend well. Stir the dry ingredients into the wet ingredients, mixing just until incorporated.
In the bowl of an electric mixer fitted with the whisk attachment, whip the egg whites on medium-high speed until foamy. Continue beating, gradually adding in the remaining 6 tablespoons of sugar, until stiff, glossy peaks form. Add a quarter of the egg white mixture to the bowl with the batter to lighten. Gently fold in the remaining egg whites until the batter is smooth and no streaks remain.
Divide the batter between the prepared cupcake liners, filling each about three-quarters full. Bake until the cupcakes spring back when lightly pressed, about 16-18 minutes, rotating the pans halfway through baking. Allow to cool in the pan about 5 minutes, then transfer to a wire rack to cool completely.
To make the filling, combine the marshmallow creme and butter in a mixing bowl and beat on medium-high speed until smooth. Blend in the confectioners’ sugar and salt and whip until the mixture is light and fluffy, about 1-2 minutes more. Transfer the majority of the filling mixture to a pastry bag fitted with a medium round tip, reserving ½ cup in the mixing bowl. (The tip I use is not labeled, but the opening is just under ½-inch in diameter.) Blend in the cream to the reserved filling mixture to make it smooth for piping the swirl on top of the cupcake. Transfer this to another pastry bag fitted with a small round tip (I used Wilton #3). Chill until ready to use.
Fill the cupcakes by inserting the pastry tip directly into the center of each cupcake and squeezing in a dollop of the marshmallow mixture. (See this post for a visual – an injector tip will not work for this recipe because the filling is too thick.) If excess filling comes out of the top, just scrape it off with a knife so the top of the cupcake is essentially smooth.
Make the chocolate glaze by placing the chopped chocolate in a small bowl. Bring the cream to a simmer in a small saucepan or the microwave. Pour the hot cream over the chocolate and let stand 1-2 minutes. Whisk together until a smooth ganache forms. Let cool briefly, about 5 minutes. Take the cupcakes one at a time and invert them, dipping the top into the chocolate glaze. Transfer them to a wire rack or baking sheet and let the glaze set. (You can speed this process up by chilling in the refrigerator.) Once the glaze is set, use the reserved portion of marshmallow frosting to pipe a uniform swirl across the top of each cupcake. Store in an airtight container until ready to serve.