1. Mix all ingredients for the base together.
2. Pour it on a cookie sheet evenly and pat them down.
3. Cook the base at 325º F for 12-14 minutes or until the edges are golden brown. Let it cool for 10 minutes.
4. During that time, prepare the filling. In a big bowl, whip the eggs with the sugar until it stiffens a little.
5. Add the juice and the zest while still whipping.
6. In a small bowl, mix the flour with the baking powder, then add it to the other ingredients. Mix well.
7. Pour the filling onto the base and cook for 25 to 30 minutes or until the centre solidifies.
8. Let cool, cut into squares and serve.