Beat the butter and sugar together for 5 minutes until light and fluffy.
Add the eggs one at a time, with a spoonful of the flour mixture between each addition.
Add the chopped and ground almonds, the remaining flour, the lemon zest and vanilla extract. If the mixture is too thick add a little milk or yoghurt until it is a dropping consistency.
Mix the amaretti biscuits and amaretto together in a separate bowl.
Pour half the mixture into tin and level the surface with a palette knife until smooth. Place the crushed amaretti biscuits on top. Cover with the rest of the mixture and sprinkle the top with sugar. Bake in the oven at 160°C for one hour or until a toothpick inserted in the middle of the cake comes out clean.