For the crust: Preheat the oven to 350 degrees F. Grease a 9-by-13-inch pan with butter.
Stir together the flour, granulated sugar and salt in a bowl. Add the butter to the bowl and use a pastry cutter to cut it all together until the mixture resembles fine crumbs. Press into the prepared pan and bake until golden around the edges, about 20 minutes.
For the filling: Stir together the granulated sugar and flour in a bowl. Crack in the eggs and whisk to combine. Add the orange zest and juice and mix until combined. Pour over the crust and bake until the filling is set, about 20 minutes.
Allow to cool in the fridge for a minimum of 2 hours, then sift powdered sugar over the top before cutting into 16 pieces.