1Heat oven to 350°F. Spray bottom only of 9-inch square pan.
2For Crust, in medium bowl, stir together 1 1/2 cups flour blend, 1/2 cup granulated sugar and the xanthan gum. Cut in butter, using pastry blender (or pulling 2 table knives through mixture in opposite directions), until mixture looks like coarse crumbs. Add water, 1 teaspoon at a time, tossing with fork, until crumbly. Press dough in bottom of pan. Bake 20 to 25 minutes or just until set.
3For Filling, in medium bowl, stir together all ingredients except powdered sugar with whisk until blended. Pour over hot crust.
4Bake 23 to 28 minutes longer or until filling no longer jiggles when pan is moved. Cool completely on cooling rack, about 2 hours. Cut into 4 rows by 4 rows. Sprinkle with powdered sugar before serving. Cover and refrigerate any remaining lemon squares.