Stack two similarly-sized cookie sheets on top of one another and line the top one with parchment paper (this helps keep the bottoms of the cookies from browning too much — if you don't have two cookie sheets, you don't have to stack them. Just bake on a higher rack and/or keep a close eye on the cookies as they bake to watch for browning.)
Sift the almond flour into a large bowl, then whisk in the sugar, salt, and instant espresso powder until fully combined.
2 1/4 c almond flour, 1 c granulated sugar, pinch salt, 2 tsp instant espresso powder
In the bowl of a stand mixer fitted with a whisk attachment (or in a large bowl, using a hand mixer), whisk egg whites, cream of tartar, and vanilla extract at medium-high speed until soft peaks form.
2 lg egg whites, 1/4 tsp cream of tartar, 1/2 tsp vanilla extract
Add beaten egg whites to the almond flour mixture, and mix until there are no dry bits and a sticky dough forms. If it gets difficult to mix, use your hands to make sure everything is mixed thoroughly.
Using a small cookie scoop or your hands, form dough into 1-inch balls. Roll in powdered sugar and place 1 inch apart on prepared cookie sheet.
powdered sugar for rolling
Bake for 30-35 minutes, until tops are cracked and bottoms are beginning to brown (if you're not using a doubled-up cookie sheet, they may bake faster – check at 25 minutes).
Let cool on cookie sheet for 5 minutes, then move to a cooling rack to cool completely. Store in an airtight container at room temperature up to 3 days or freeze in an airtight container up to 2 months.