Preheat oven to 300 degrees F. Stack two matching, heavyweight, light to medium colored cookie sheets one inside the other (stacking two cookie sheets together keeps the bottoms of the cookies from getting too brown). Line with parchment paper or a silicone baking mat.In a large bowl, whisk together almond flour, sugar, baking powder, and salt 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 ube 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 parchment or silicon-lined baking sheets, leaving 1 inch of space between cookies.Bake for about 26 to 30 minutes until tops are cracked and bottoms are just barely starting to brown (if you are not using doubled cookie sheets the bottoms will brown much quicker, so watch them closely). Remove from oven; let cool a few minutes, then transfer to wire racks to cool completely.Cookies will keep at room temperature in an airtight container for up to 5 days.