Preheat oven to 355 degrees F (180 degrees C)
Line the base and sides of an 8 inch (20cm) round springform cake pan with parchment paper.
Using a mixer, blend 3.5 ounces (100g) of the sugar with the lemon zest until combined.
Add the butter and vanilla and beat until mixture is pale and creamy (about 5 minutes).
Add egg yolks one at a time beating well after each yolk.
Add the almond flour and beat until well combined.
Fold in the ricotta and the limoncello.
Beat the egg whites in a clean bowl until soft peaks form.
Gradually add the remaining sugar to the egg whites and beat until stiff peaks form.
Mix a third of the egg whites into the cake batter until just combined and then gently fold in the remaining egg whites.
Pour the batter into the prepared cake pan. Smooth the top and sprinkle with the almonds slices.
Bake for about 40-45 minutes until firm to the touch and a toothpick comes out clean.
Allow the cake to cool completely in the pan before removing.
Dust with icing sugar before serving.