Preheat the oven to 350º. Line cupcake pans with 24 paper liners.
In a large heat proof bowl, combine the cocoa and hot water; whisk until the cocoa is dissolved. Let cool 5 minutes.
In a medium bowl, beat the eggs. Whisk in the granulated sugar, oil, vanilla and salt until well combined. Whisk in the cocoa mixture.
In a separate medium bowl. Add the flour, milk, baking powder and baking soda. Whisk until just combined, then add to the egg mixture and mix just until incorporated. Divide among the cupcake liners and bake until the cupcakes have risen and a toothpick or cake tester inserted through the center of one comes out clean, about 18 minutes.
Make the frosting: Fill a medium saucepan with 1 inch of water and bring to a low simmer. In the bowl of a stand mixer, add all of the frosting ingredients; whisk well to combine. Place the bowl atop the simmering water and let cook, whisking frequently, until the egg whites feel warm and the sugar appears dissolved, about 2 minutes.
Transfer the egg white mixture to the stand mixer and whisk (with a whisk attachment) on high speed until soft peaks form, 4 to 5 minutes.
Transfer the marshmallow frosting to a piping bag fitted with a round attachment. Pipe a thick layer of frosting onto each cupcake. Garnish with shaved chocolate and a few marshmallow bits.