Preheat your oven to 325 degrees.
Grease a 9 inch springform pan with butter or baking spray.
Using a grater (a cheese grater with medium sized holes works fine) grate the almond paste into a mixing bowl. Add the eggs, lemon zest, sugar, vanilla and limoncello to the bowl. Using an electric mixer beat on medium speed until all the ingredients are combined.
Add the ricotta and beat on medium speed until smooth.
Pour into the prepared pan and bake for one hour 30 minutes or until the top of the cake is golden and the middle has set.
Cool for 20-30 minutes before opening the spring.
Store in the refrigerator.