For the macarons:
Line a baking sheet with high quality parchment paper or a silpat. Fit a large pastry bag with a 1/2-inch plain tip.
Combine the powdered sugar, almond flour, cocoa powder, and salt in a food processor. Pulse several times to combine. Continue to process until the mixture is very fine, about 30 seconds. Sift with a fine strainer into a large bowl.
Place the egg whites in the clean bowl of a stand mixer fitted with the whisk attachment. Beat until fluffy and white, about 1 to 2 minutes. Gradually add the sugar and continue beating until stiff shiny peaks form, about 1 to 2 minutes more. Do not overwhip.
Fold the dry ingredients into the egg whites in three separate additions. Do not overmix. Fold just until there are no large lumps or white streaks left.
Transfer the batter to the pastry bag. Pipe out 1 1/4-inch rounds about 1-inch apart onto the baking sheet. Rap the sheets against the counter until the macaron surfaces become flatter, helping to create the foot. Let the rounds sit at room temperature for at least 30 minutes, or until a matte skin forms on top of the macarons and they are no longer sticky to the touch. Meanwhile, arrange an oven rack to the middle position and preheat the oven to 350°F (if your oven runs hot, set it to 325°F).
Bake for about 14 minutes, or until the macarons are smooth, firm, and dry with “feet,” rotating the sheet halfway through. Do not overbake. Transfer the sheet to a cooling rack and cool completely.
For the ganache:
Place the chocolate and espresso powder in a large bowl. Heat the cream in a small saucepan over medium heat until it just comes to a boil. Pour the hot cream over the chocolate. Let stand for one minute then add the butter and stir the mixture until the chocolate is melted and the mixture is smooth. Chill the ganache until it is firmed but still spreadable, about 30 minutes.
Match macarons of similar size. Place the ganache in a piping bag fitted with a plain tip. Pipe out about a teaspoon of ganache on the center of the macaron half. Top with the other macaron and press down gently. Repeat for remaining macarons. Store the macarons in an airtight container in the refrigerator. Macarons actually taste best the day after they’re baked.