Preheat oven to 335 degrees F.

Melt butter, and set aside to cool.

Line a 13 x 9 inch pan with foil. (I use a 3 fool piece, folded in half. Then I fold each end over about an inch to create reinforced handles.

Spray the foil with non-stick baking spray.

Whisk gluten-free flour, powdered sugar, cornstarch, and salt until combined. Add cooled butter, and mix to form dough. Press into foil lined pan.

Bake at 335 degrees for 25 minutes.

While crust is baking, wash lemons, and remove 1 tablespoon zest. Squeeze ¾ cup juice.

Whisk together eggs, sugar and cornstarch. Then add zest and juice and whisk until completely incorporated.

Remove crust from oven, and slowly pour lemon mixture on top. Carefully return to oven and bake another 25 minutes.

Remove from oven, and allow to cool 10 minutes on a cooling rack. Using the reinforced ends of the foil, gently lift from pan, and cool the rack. When completely cool, return to baking pan, cover tightly, and refrigerate several hours or overnight.

Just before serving, sift powdered sugar over top, and cut into squares.

Serve plain or garnished with whipped cream, ice cream, or berries.