To make the cupcakes, preheat the oven to 350°F (175° C). Line 2 cupcake pans with 16 cupcake liners.
In a small bowl, whisk together flour, sugar, cocoa powder, baking soda, baking powder and salt.
In a medium bowl, whisk the egg, buttermilk, melted butter and vanilla extract until well combined.
Add the dry ingredients and fold the batter until just combined.
Divide the batter into the prepared cupcake cups, about 2/3 of the way full. (TIP:Use a large cookie scoop, to measure the batter. 1 large cookie scoop full of batter (about 1/4 cup) in each cupcake cup.)
Bake for 15-18 minutes, or until the inserted toothpick comes out clean. Cool completely.
To make the frosting, melt chopped chocolate in a microwave in 30-second increments at 50% power, stirring well every 30 seconds, until nice and smooth. (Alternatively, you can use a double-boiler.) Cool slightly.
In a mixing bowl with whisk attachment, whip the heavy cream with the sugar until hard peaks form.
Dissolve cocoa powder in a small bowl with hot water and stir it into the melted chocolate.
Fold in half of the whipped cream until almost smooth.
Then add the remaining whipped cream and mix well until no white streaks remain.
Refrigerate until cupcakes are cooled completely.
To frost the cupcakes, I used a large cookie scoop to scoop the frosting and topped each cupcake. Alternatively, you can transfer the frosting into a piping bag fitted with desired tip. And pipe the frosting on top of each cupcake. Dust with powdered sugar and decorate with chocolate macarons, if desired.
Store frosted cupcakes in the fridge for up to 3 days.