Preheat your oven to 350°F (175°C) and line a muffin tin with cupcake liners. In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt.

In a large bowl, beat the butter and sugar together until light and fluffy. Add the eggs one at a time, mixing well after each addition, followed by the vanilla extract. Gradually add the dry ingredients to the wet mixture, alternating with the buttermilk. Begin and end with the flour mixture. Finally, stir in the hot water until the batter is smooth.

Divide the batter evenly among the cupcake liners, filling each about two-thirds full. Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted into the center of a cupcake comes out clean. Once baked, remove the cupcakes from the oven and let them cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

In a heatproof bowl set over a pot of simmering water, whisk together the egg whites, sugar, and cream of tartar. Continue to whisk until the sugar is dissolved and the mixture is warm to the touch. Remove the bowl from the heat and beat the egg white mixture on high speed until stiff, glossy peaks form. Beat in the vanilla extract.

Once the cupcakes are completely cool, pipe or spread the marshmallow topping onto each cupcake.

Store the cupcakes in an airtight container at room temperature for up to 3 days. If refrigerated, allow them to come to room temperature before serving for the best flavor and texture.