Preheat oven to 325 degrees Fahrenheit. Butter a 9-inch springform pan. Line the bottom of the pan with parchment paper.
Using an electric mixer, beat the butter and sugar together until incorporated. Mix in the ricotta, vanilla and lemon zest.
Scrape down the sides of the bowl, then add the egg yolks, one at a time, continuing to beat until very light and creamy. Add in the limoncello, almond flour and baking powder and beat to combine.
In a separate clean chilled bowl, beat the egg whites with an electric mixer until stiff peaks form. Gently fold the egg whites into the cake mixture. (Don’t worry if white streaks remain – they will disappear once in the oven.)
Pour the mixture into the prepared springform pan. Smooth the top with a spatula or spoon. Sprinkle with the sliced almonds. Bake for 40-45 minutes or until firm yet slightly springy to the touch.
Allow to cool completely. (It will fall slightly.) Dust with the powdered sugar and serve!