Preheat oven to 170°F (75°C). Spread sifted nut flours on a parchment-lined baking sheet and bake for 30 to 45 minutes or until dry and toasty. Let cool completely then transfer to a large bowl.
Increase oven temperature to 300°F (150°C).
Stack two matching, heavyweight, light to medium colored cookie sheets one inside the other and line with parchment paper or a silicone baking mat.
Add sugar and salt to cooled nut flours and whisk until evenly incorporated.
In a mixing bowl or the bowl of a stand mixer fitted with the whisk attachment, whisk egg whites and lemon juice until they hold soft peaks.
Add beaten egg whites and pistachio extract to dry ingredients and stir until mixture forms a soft, sticky dough, kneading with your hands if necessary.
Lightly dust your hands with powdered sugar. Use a small cookie scoop to portion dough into 1-inch balls. Roll into a smooth ball, then roll in powdered sugar. Arrange on prepared baking sheet, leaving 1 inch of space between cookies.
Bake for 30 to 35 minutes until tops are cracked and bottoms are just barely golden. Remove from oven; let cool a few minutes, then transfer to wire racks to cool completely.