Preheat oven to 350 degrees F. Line a 8x8x2-inch square pan with foil. Lightly grease the foil with nonstick cooking spray.
Pulse together the the gluten-free flour, powdered sugar, salt, and baking powder in the bowl of a food processor. (Or whisk together in a large bowl.) Add the butter. Run the processor until a dough forms, about 30 seconds. (If mixing by hand, cut butter in with a pastry cutter.)
Press dough evenly into prepared pan. Bake until golden brown, about 25 minutes. Remove pan from oven, leaving the oven on, while you prepare the filling.
For the filling: Stir together the granulated sugar and cornstarch. Rub in the lemon zest. Add the eggs and egg yolk. Whisk until smooth. Add the lemon juice and stir. Pour mixture over baked crust.
Bake until filling sets, about 30 minutes. Remove pan from the oven and allow the bars to cool in the pan. When cool, sprinkle with powdered sugar, optional.