Preheat oven to 350 degrees. Combine 2 cups flour with 1/2 cup powdered sugar and set aside. Pour melted butter into a 9×13-inch baking dish (metal preferred). Stir in flour and powdered sugar mixture. With a fork, mix until well combined, creating a dough. Press evenly with your hand to form a crust in the bottom of the pan. Bake for 20 minutes. Meanwhile, beat together eggs and bottled lemon juice until fully combined. Just before the crust comes out of the oven, to the egg mixture add 2 cups granulated sugar, 4 tablespoons flour, salt and baking powder. Blend with a electric mixer on medium until combined. Pour over hot crust. Bake for another 25 minutes. Let cool. Cut into squares and dust with powdered sugar to serve. Squares can be covered with foil and refrigerated or made ahead and frozen in a well-sealed air-tight container. Makes approximately 24 squares.