Beat egg whites with granulated sugar, salt, and optional extract until soft peaks form. Gently fold in dry ingredients.
Fill pastry bag fitted with 1/4" diameter tip. Pipe batter in 1.25" rounds on parchment-lined pastry sheets, leaving 1" between. Let sit for 30 minutes.
Bake at 375°F for 5 minutes; lower temperature to 325°F and bake 10 min more. Remove from oven and let cool.
Fill matched cookies with filling, such as lemon curd.