1. In a bowl, blend together softened butter, sugar and flour (first 3 ingredients).
2. Preheat oven to 350F (180C). Line a 9-by-13-inch baking dish with parchment paper (make sure it's large enough to cover the whole dish), or just brush it with oil.
3. Press the dough into the bottom of the dish.
4. Bake in the preheated oven for 15-20 minutes, until very lightly golden.
5. Meanwhile, in a bowl, whisk together eggs, lemon juice, lemon zest, orange zest, the remaining sugar and flour.
6. Pour over the baked crust and bake for another 20-25 minutes, until the filling is set.
7. Let cool at room temperature. Cut into squares or triangles. Dust with powdered sugar and serve.