FOR THE FILLING:
Preheat oven to 350° F. Grease and flour a 9 x 13-inch glass baking dish or line the bottom and sides with parchment paper, leaving an overhang on the sides.
PREPARE CRUST:
In a large bowl, stir together melted butter, flour and confectioners sugar until completely combined.
Press the mixture into the prepared baking dish. I like to use the flat bottom of a measuring cup to gently press the crust into an even layer. Bake until golden brown, about 15-20 minutes.
While the crust is in the oven, make the filling. In a large bowl, whisk together the sugar, flour and baking powder. Add the eggs, lemon juice and lemon zest; whisk until completely combined.
Pour the filling over the hot crust. Return to the oven and bake for 25 minutes.
Cool completely on a wire rack. Dust with additional powdered sugar.
Slice into squares and serve.