For the cupcakes:
Heat the oven to 350°F and arrange a rack in the middle. Line 2 (12-well) muffin pans with paper liners; set aside.
Bring the water to a boil over high heat in a medium saucepan. Remove from the heat, add the chocolate and cocoa, and whisk until smooth; set aside to cool for 15 minutes.
Meanwhile, whisk together the flour, baking soda, and salt in a medium bowl until combined; set aside.
Place the butter and sugar in the bowl of a stand mixer fitted with a paddle attachment and beat on medium speed until fluffy and light in color, about 5 minutes. Stop the mixer and scrape down the sides of the bowl and the paddle with a rubber spatula.
Return the mixer to medium speed. Add the eggs 1 at a time, letting each incorporate fully before adding the next. Add the sour cream and vanilla and beat until just combined (the mixture will look slightly curdled). Stop the mixer and scrape down the sides of the bowl and the paddle.
Add a third of the reserved flour mixture and turn the mixer to low speed, mixing until the flour is just incorporated. Add half of the cooled chocolate mixture and mix until just incorporated. Continue with another third of the flour mixture and the remaining chocolate mixture, and end with the remaining flour, mixing until all of the ingredients are incorporated and the mixture is smooth. Stop the mixer, remove the bowl, and stir in any remaining flour streaks by hand, making sure to scrape the bottom of the bowl.
Fill the muffin wells about three-quarters of the way. Place the muffin pans side by side in the oven and bake for 8 minutes. Rotate the pans front to back and side to side and bake until a toothpick inserted into the center of the cupcakes comes out clean, about 7 to 8 minutes more.
Place the pans on wire racks and let them cool for 10 minutes. Remove the cupcakes from the pans and cool completely on the racks before frosting, about 40 minutes.
For the marshmallow frosting:
Place the cooled cupcakes on a baking sheet (they can be touching) and set them aside. Heat the broiler to high and arrange a rack in the middle.
Fill a medium saucepan with 1 inch of water and bring it to a simmer over medium heat.
Place the egg whites, sugar, cream of tartar, and salt in the clean bowl of a stand mixer and whisk by hand to combine. Nest the bowl over the saucepan, making sure the bottom of the bowl is not touching the water. Heat the egg white mixture, whisking constantly, until the sugar has dissolved and the mixture is hot to the touch (about 120°F on an instant-read thermometer), about 6 minutes.
Transfer the bowl to a stand mixer fitted with the whisk attachment. Turn the mixer to medium and whisk for 1 minute. Increase the speed to high and whisk until stiff, glossy peaks form, about 5 minutes more. Add the vanilla and whisk until just incorporated, about 1 minute.
Transfer the frosting to a large piping bag fitted with a 1/2-inch star piping tip and pipe it onto the cupcakes. Broil until the tops are starting to brown, about 20 to 30 seconds—watch carefully to avoid burning. (Alternatively, you can use a kitchen torch.) Serve immediately or store in an airtight container at room temperature for up to 3 hours.
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