Makes 24 standard cupcakes
Preheat oven to 350 degrees.
Line two standard muffin tins with paper liners and coat liners with cooking spray.
Combine chocolate and cocoa in a medium bowl. Pour 1 1/2 cups boiling water over chocolate mixture and whisk until smooth; set aside. Sift together flours, baking soda, and salt into a medium bowl; set aside.
Place butter in a large bowl and beat with an electric mixer on medium-high speed until creamy, about 1 minute. Add dark brown and granulated sugars and beat on high until light and fluffy, about 3 minutes. Stop mixer and scrape down bowl with a rubber spatula. Add eggs, one at a time, beating 30 seconds on medium-high after each addition. Reduce speed to medium and add sour cream and vanilla. Beat until combined. Stop mixer and scrape down bowl. With mixer on low, add about one-third the flour mixture, followed by about one-half the chocolate mixture. Repeat, ending with flour mixture and beating until just combined. Stop mixer and scrape down bowl. Stir batter gently to thoroughly combine. Divide batter evenly among muffin tins, filling them about three-quarters full. Smooth tops with a spatula.
Bake until a skewer inserted in center of cupcakes comes out clean, about 20 minutes. Cool cakes in their pans set on a wire rack for about 15 minutes. Invert pans, one at a time, onto a large plate, and then invert cupcakes onto the rack to finish cooling.
Once cool, use an apple corer or a melon baller inserted into the middle of each cupcake to remove a small tubular portion, pressing about halfway down center. Reserve this portion of cake to reseal cupcake after filling. Fit a pastry bag with a round tip or snip the end of a disposable pastry bag, then fill with Marshmallow Filling. Pipe some filling into the hollowed-out portion of cakes to just fill cavity. Seal cupcakes with reserved cake pieces.
To finish cupcakes, use a pastry bag fitted with a large plain tip to pipe some of the Seven Minute Frosting onto each cupcake: Hold the piping bag over each cupcake with the tip just above the top of the cake; squeeze the bag from the top, releasing pressure once you have created a ghostly shape.
Once these cupcakes are frosted, decorate as desired. They are best eaten the day they are made.