Preheat oven: Preheat oven to 350F. Prep baking sheet with parchment paper and baking spray.
Combine dry ingredients: In a large bowl, combine the almond flour and pistachio flour with the granulated sugar.
Whisk egg whites: In a large bowl, whisk the egg whites and the salt until soft peaks form. You could use an electric mixer, a hand whisk or a fork. Gently whisk in the almond extract.
Make the dough: Start to add the almond/pistachio flour mixture into the egg whites in portions, stirring with a wooden spoon after each addition.
Add in the egg yolks: Stir in the egg yolks. The mixture will seem dry, but keep stirring. You could even use your hands to work the dough together. It should be tacky, almost like an almond paste. Since they are gluten-free, you won’t get tough cookies from overworking the dough.
Get ready to roll cookie balls in confectioner's sugar: Place some confectioner’s sugar in a bowl or a plate (this is to roll the cookie balls into).
Roll out the cookie balls: Using a tablespoon, a spoon, or a small scoop, take out portions of the dough and use your hands to form the dough into small balls. The dough is very sticky, so you could prep your hands with either baking spray or lightly dust them with confectioner’s sugar to help roll out the balls.
Cookie size: The cookies should be small, so you should roll out about 1 ½-2 dozen cookie balls. It’s about 1-inch round (if you have a scale, you could measure each ball to be precise in the size/weight. I make so many that I can tell the size by eyeing it and feeling the weight of each cookie).
Coat the cookies: Roll the cookie balls coating the cookies very well with the confectioner's sugar.
Place cookies on baking sheet: Arrange the cookie balls on your prepped baking sheets.
Flatten gently: Gently flatten the cookie balls with the bottom of a glass or even a knife. Press a half of pistachio on top of each cookie (or some ground pistachios). The cookies will crack a bit on the sides, and that is fine.
Bake cookies: Bake for 15-20 minutes. The cookies will slightly rise and form some cracks. Check on them at 15 minutes (every oven is different). They should be soft and LIGHTLY golden. Take care to not overbake them. If you rolled them smaller, check on them at the 10 minute mark.
Cool down cookies: Set them aside to cool for 5 minutes, then transfer to a rack to cool completely.