Crust: cream the butter and sugar until light and fluffy. Combine the flour & salt, and slowly add to the butter/sugar mixture while the mixer is on low. Mix just until it is combined. Dump the dough into your 9×13-inch pan, and using hands dusted with gluten-free flour, press the dough evenly over the bottom of the pan, building up a 1/2-inch edge on all sides.
Bake in preheated oven for 20 minutes, or until very lightly browned. Remove from oven and cool on wire rack.
Filling: whisk together the eggs, sugar, lemon zest, lemon juice, and gluten-free flour. Pour over your pre-baked crust and bake for 30-35 minutes, until the filling is set. Let cool to room temperature.
Using a sifter or wire sieve to dust the top of the bars with icing sugar. Cut into squares, and serve.