Grind the coffee grounds into fine powder using a spice grinder or food processor.
Sift the powdered sugar, almond flour, and powdered coffee grounds into a bowl. Mix until combined and set aside.
Next make your meringue. Use a hand mixer to whisk your eggs until soft peaks and small bubbles appear. Add the cream of tartar and 1/3 of the granulated sugar. Continue whisking and adding the sugar little by little. Whisk the eggs until you get a meringue with stiff peaks and a shiny white color. The best way to know if your meringue is done is to invert the bowl. If the meringue doesn't fall out, the meringue is done.
Sift the almond flour mixture from step 1 into the meringue. Use a spatula to fold the almond mixture into the meringue. Be careful not to deflate the meringue too much or over mix. The mixture is adequately combined once it has the consistency of molten lava. The best way to test if the mixture is done is to draw an 8 with the mixture.
Fill a piping bag with the mixture. Place a silicone mat on a baking sheet. Pipe circular discs that are ~1 inch in diameter. It's best to use a silicone mat with a macaron template to guide your piping.
After piping, smack your baking sheet on a flat surface 2 or 3 times to get rid of air bubbles. Let the macarons rest for 40 minutes at room temperature. Resting will allow the macarons to form a skin, so when you touch it, no batter comes off.
Preheat the oven to 300 degrees F.
Once the macarons have rested and formed that skin on the outside, bake them for 13 minutes in the center of the oven.
Cool macarons for at least 30 minutes after baking.
While the macarons are resting, making the filling. Combine butter, condensed milk, and vanilla extract in a bowl. Use a hand mixer to whisk until a fluffy, smooth mixture forms.
Add the powdered sugar and whisk until combined.
Now it's time to put together your macarons! Use a piping bag or Ziploc bag to pipe the filling onto the flat side of 1 cookie. Put another cookie, flat side down, on top of the filling. Repeat this process until the cookies or the filling is used up.