Preheat the oven to 350 degrees F. Line two 12-cup cupcake pans with foil paper liners (foil liners are sturdier, as this batter is runny).
Place a large sifter or a sieve in a large mixing bowl. Add the flour, cocoa, baking powder, baking soda, and salt and sift.
Add the sugar and whisk until combined. In another large bowl, whisk the eggs, milk, oil, and vanilla together.
Gradually add the wet ingredients to the dry ingredients and whisk until there are no lumps and the batter is smooth.
Carefully pour in the boiling water and stir until combined. (Watch the little ones with the hot water!)
Pour the batter into the prepared cupcake pans and don't fill the liners more than halfway.
Bake the cupcakes for 20 to 25 minutes, or until a wooden skewer inserted in the center comes out clean, and the cake bounces back when lightly pressed.
Remove the cupcakes from the oven and let them cool on a wire rack.
While the cupcakes are baking, make the Silky Marshmallow Icing according to directions.
For the Silky Marshmallow Icing:
Fill a medium saucepan with about 1 1/2 inches of water, bring to a boil, and then lower the heat to maintain a gentle simmer.
Place the water, egg whites, sugar, cream of tartar, salt, and vanilla in a metal mixing bowl. (Use the mixing bowl of a stand mixer if you have one.) Place the bowl on top of the saucepan. Make sure the bottom of the bowl doesn't touch the water.
Whisk the mixture constantly for 4 minutes or until it reaches a temperature between 160 degrees and 165 degrees F if using a candy thermometer. The sugar will have dissolved, and the mixture will be opaque, bubbly, and very warm to the touch.
Carefully transfer the hot bowl to the electric mixer. Using a whisk attachment, beat the mixture on high speed for 4 minutes or until it's white and fluffy with stand-up peaks.
Once the cupcakes have cooled, ice the tops with the Silky Marshmallow Icing.