Start with the ganache, which should be made the day before. Bring the heavy cream to a boil. Remove from heat and add the chocolate in pieces, stirring well. Add the butter, mix, pour into a bowl, and refrigerate overnight so it sets and isn't runny.

On the day you assemble, make the shells. Beat the egg whites until they form soft peaks. Gradually add the sugar in three parts as you continue beating. The meringue is ready when it forms stiff peaks. Add the cocoa powder and beat until fully incorporated.

Next, blend the almond flour and powdered sugar together until you have a fine powder.

Add this powder to the chocolate mixture and fold gently. Use a spatula to press and smooth the batter until it flows in a ribbon when lifted.

Transfer the batter to a piping bag fitted with a plain tip and pipe the macarons in staggered rows. Preheat your oven to 290°F (145°C) with convection. Let the shells sit for about 15 minutes to form a skin, then bake for 12 minutes and let cool.

You should have shells that look like this:

Take the ganache out of the refrigerator and transfer it to a piping bag with a plain tip. Pair up the macaron shells by size, then fill them with the ganache.