Combine almond flour, powdered sugar, and espresso powder by sifting them together into a bowl and set aside.
Using a stand mixer with a whisk attachment, whip the egg whites on medium speed until foamy; add the cream of tartar.
Increase mixer speed to medium-high and slowly blend in granulated sugar to avoid deflating the meringue; then whisk at high speed until stiff peaks form.
Carefully fold half of the dry ingredients into the meringue with a spatula until blended, then fold in the remainder until the batter flows like lava and can form a figure 8 without breaking.
Prepare a baking sheet lined with a silicone mat or parchment; fill a piping bag with batter and pipe 1-inch rounds straight down onto the mat.
Lift the baking sheet and firmly drop it on the counter 3 to 4 times to remove air bubbles; pop any bubbles with a toothpick for a smooth surface.
Let the piped batter rest at room temperature for 30 to 40 minutes until a matte skin forms and the surface no longer feels sticky.
While resting, preheat the oven to 300°F (150°C).
Bake each sheet in the oven's center rack for 15 minutes or until the macarons do not move when gently nudged by their feet.
Allow macarons to cool completely on the tray before carefully peeling them off; pair shells by size and flip one to prepare for filling.
In a stand mixer with a paddle attachment, beat softened butter, powdered sugar, espresso powder, and vanilla extract until smooth and creamy.
Whip the buttercream on high speed for 3 to 4 minutes until it is light and fluffy.
Pipe a dollop of buttercream onto one macaron shell and sandwich it with the matching shell.
Store the assembled macarons in an airtight container and refrigerate for 12 to 24 hours to develop flavors; keep refrigerated up to 5 days.
Before serving, remove from fridge and allow macarons to come to room temperature for about one hour for the best texture and softness.
