In a food processor, place macaron shells and process them until finely ground. (You may need to occasionally pulse it to get the stubborn chunks to break down.)
Add cocoa powder, espresso powder, baking powder and salt into the macaron flour. Pulse everything a few times until well combined.
Place chocolate chips in a microwave-safe bowl and melt the chocolate in a microwave. Place the bowl with chocolate chips for 1 minute at 50% power, stir well and microwave for 30 seconds. Stir well and microwave again for 15 seconds, if needed. Let the melted chocolate cool slightly.
In a mixing bowl with paddle attachment, beat the butter until fluffy.
Stir in eggs and beat for a minute or so on a medium speed until the mixture is almost combined. (Note: Butter and eggs won't fully combine into smooth mixture, but if they're both at the same room temperature, they'll come close.)
Add cooled melted chocolate into the butter mixture and whip on medium high speed until well combined, about a minute.
Now, add dry ingredients and mix on low speed until just dry ingredients are incorporated. The dough will be super soft, but that's totally normal.
Preheat the oven to 350°F (177°C). And line 2 baking sheets with parchment paper or silicone mat.
Chill the dough for 20 minutes in the refrigerator. (No more than 30 minutes, or the mixture will harden too much.)
Using a medium cookie scoop, drop 1.5-inch balls on prepared baking sheets, leaving about 2-inch space between the cookies. Bake the cookies, one sheet at a time, for 12-13 minutes, or until the edges are set and slightly soft in the middle.
Let the cookies cool on a baking sheet for 5 minutes and then transfer them onto a cooling rack to cool completely.
Store the cooled cookies in an airtight container for up to 5 days.