For the cupcakes: Preheat the oven to 350˚. Line two 12-cup muffin pans with paper liners. Melt the butter in a medium saucepan over medium heat and stir in the cocoa powder. Pour in the boiling water and stir to combine. Let boil for 15 seconds, then remove from the heat and set aside.
In a medium bowl, stir together the flour, sugar and salt; set aside. In a small bowl, whisk together the eggs, buttermilk, vanilla and baking soda. Pour the cocoa mixture over the flour mixture and stir until halfway combined, then pour in the egg mixture and stir until the batter comes together.
Fill the muffin cups two-thirds full (a scant 1/4 cup per cupcake) and bake until a toothpick inserted into the centers comes out clean, about 20 minutes. Remove from the oven and let cool in the pans for about 5 minutes, then remove to a rack and let cool completely before decorating.
For the meringue filling: Whisk the granulated sugar, cream of tartar and egg whites in a large metal bowl until smooth. Place over a pan of simmering water (don’t let the bottom of the bowl touch the water). Heat, stirring, until the mixture is warm and the sugar is dissolved, 2 to 3 minutes. Remove the bowl from the pan and beat with a mixer on medium-high speed until the meringue is cool and forms stiff glossy peaks, 4 to 6 minutes. Beat in the vanilla.
Scoop a heaping 2 tablespoons meringue on top of each cupcake and spread into a small mound.
For the glaze: Put the chocolate and butter in a medium heatproof bowl. Heat the heavy cream in a small saucepan until steaming and bubbling around the edges. Pour over the chocolate mixture and shake the bowl gently to settle the chocolate into the cream. Let stand for 5 minutes and then stir until smooth. One at a time, dip the top of each cupcake in the glaze, moving it around until the meringue is completely coated. Let any excess glaze drip off and set the cupcakes on a rack.
Stir the confectioners’ sugar with 2 teaspoons of hot water in a small bowl until thick and smooth. Transfer to a small resealable plastic bag and snip off a corner. Pipe loops of white icing onto the top of each cupcake. Set aside to firm up, at least 1 hour.