Preheat the oven to 180°C/350°F. Line a 20cm/8-inch baking sheet with parchment paper.
For the crust: Put the flour, white sugar and salt in a blender or food processor and mix. Add the butter and process (you can also use your hands) until the dough comes together. Press the dough lightly into the baking sheet with your fingers. Bake for 20 minutes.
In the meantime, mix all the filling ingredients in a bowl and reserve.
Take the tray out of the oven and cover the baked crust with the filling. Bake for another 20 minutes, or until the filling is set. Let it cool at room temperature and then refrigerate for 4 hours.
Cut into squares and dust with powdered sugar right before serving.