1. Preheat oven to 350°.
2. Line an 8-inch square metal baking pan with foil, allowing foil to extend over edge of pan; coat foil with cooking spray.
3. Weigh or lightly spoon potato starch, brown rice flour, and almond meal flour into dry measuring cups; level with a knife. Place potato starch, brown rice flour, almond meal flour, brown sugar, butter, 1/4 teaspoon salt, and almond extract in a food processor; process until well blended. Press mixture into bottom of prepared pan. Bake at 350° for 10 minutes; cool 15 minutes on a wire rack.
4. Wipe out food processor bowl with a paper towel. Place 1/4 teaspoon salt, granulated sugar, white rice flour, and next 4 ingredients (through eggs) in food processor; process until well blended. Pour into prepared crust.
5. Bake at 350° for 25 to 27 minutes or just until center is set. Cool completely in pan on a wire rack.
6. Lift lemon bars from pan, using foil sides as handles. Cut into 16 squares. Remove from foil; place on a platter. Dust with powdered sugar.