Grease and line a 20cm square cake pan.
Using an electric mixer, beat the butter, icing sugar and vanilla together until pale and creamy.
Add the egg. Fold in the flour by hand. Press into the base of the tin and rest in a fridge for 1 hour to firm up.
Preheat the oven to 160 degrees. Bake the base for 25 minutes or until golden brown. Set aside to cool.
For the topping combine the eggs, yolks, sugar, lemon juice and zest in the mixer. Fold in the flour.
Pour the topping over the base and bake for 30 minutes or until firm. Cool in the pan and place in the fridge to firm.