Preheat convection oven to 175°C (350°F) and prepare a 12-well muffin tin with paper liners.
Sift flour, sugar, black cocoa, baking powder, baking soda, and salt into a large mixing bowl to ensure uniform aeration.
Whisk the egg, buttermilk, vegetable oil, and vanilla extract in a separate container until emulsified.
Incorporate the wet ingredients into the dry ingredients using a whisk until just combined.
Gently stream in the boiling water while whisking continuously until the batter is thin and homogenous.
Dispense approximately 50ml of batter into each liner, filling to 60% capacity.
Bake for 18-22 minutes until the internal temperature reaches 98°C or a probe comes out clean.
Transfer cupcakes to a wire rack and allow to reach ambient temperature.
Construct the frosting by combining egg whites, sugar, corn syrup, and cream of tartar in a double boiler over medium heat.
Whisk the mixture continuously until it reaches 71°C (160°F) to ensure pasteurization and sugar dissolution.
Transfer to a stand mixer and whip on high speed until stiff, glossy peaks form and the bowl feels cool.
Pipe the marshmallow frosting onto the cooled cupcakes and optional: lightly toast with a culinary torch for 5-10 seconds.