For the cupcakes:
¾ cup unsweetened Dutch-process cocoa powder
¾ cup hot water
3 cups all-purpose flour
1 tsp. baking soda
1 tsp. baking powder
1¼ tsp. coarse salt
1½ cups (3 sticks) unsalted butter
2¼ cups sugar
4 large eggs, at room temperature
4 tsp. vanilla extract
1 cup sour cream, at room temperature

For the filling:
8 oz. bittersweet chocolate, finely chopped
1 cup plus 2 tbsp. heavy cream
½ cup fresh mint leaves
4 tbsp. unsalted butter, cut into 4 pieces, at room temperature

For the frosting:
5 large egg whites
1 cup plus 2 tbsp. sugar
Pinch of salt
1 lb. (4 sticks) unsalted butter, at room temperature
2-3 tsp. mint extract