Sift together almond flour, powdered sugar, and espresso powder into a large bowl and set aside.
In a stand mixer fitted with the whisk attachment, whisk the egg whites on medium speed until they become foamy, then add in the cream of tartar.
Turn the mixer up to medium-high and gradually add in the granulated sugar (make sure to add the sugar in very slowly so that the meringue doesn’t deflate). Once all the sugar is added, turn the mixer to the highest setting and whisk on high until stiff peaks form.
Using a spatula, gently fold in half of the dry ingredient mixture until fully combined. Then gently fold in the remaining dry ingredients. Continue to gently fold the batter until it reaches a “flowing lava” consistency. You should be able to make a “figure 8” with the batter before it breaks. As soon as you reach this consistency, stop mixing! If you over-mix the batter, the macarons won’t rise properly in the oven.
Line a large baking sheet with a silicone mat or parchment paper. Scoop batter into a large piping bag fitted with a round tip. Hold the piping bag perpendicular to the silicone mat and pipe 1 inch rounds. Pick up the baking sheet and drop it onto the counter 3-4 times to help bring any air bubbles to the surface of the macarons. Then use a toothpick to pop any visible air bubbles to make a smooth surface.
Let the macarons sit at room temperature for 30-40 minutes or until a skin forms on the surface. You should be able to lightly touch the surface without any batter being disturbed.
While the macarons are resting, preheat the oven to 300°F.
Bake one sheet at a time on the center rack of the oven for 15 minutes or when they do not move on their “feet” when touched.
Let the macarons cool completely on the baking sheet before removing them. They should easily peel off when fully cooled. Match up your shells before filling, and then turn one side over.
In a stand mixer fitted with the paddle attachment, cream together the softened butter, powdered sugar, espresso powder, and vanilla until smooth and creamy.
Beat the buttercream for 3-4 minutes on high speed until light and fluffy.
Once the macarons are cooled, pipe a dollop of buttercream into the center of the shell. Then top with the other shell.
Put the assembled macarons in an airtight container, then place them in the fridge for mature for 12-24 hours before eating for the best taste/texture. Or just eat them all immediately. You do you.