Preheat the oven to 350 degrees. Spray a 9-inch springform pan with cooking spray and dust with almond flour.
Beat the egg whites to stiff peaks.
Combine the egg yolks, almond flour, sugar, lemon zest, and 3 tablespoons limoncello in a mixing bowl and beat until smooth and pale, about 3 minutes. Take a big spoonful of the egg whites and stir it into the mixture. Fold in the rest of the egg whites until just combined.
Pour the batter into the prepared pan and bake for 40 minutes or until a toothpick inserted into the center comes out clean.
Let cool in the pan for 5 minutes then remove to a wire rack to finish cooling. Sprinkle the top of the cake with sliced almonds.
Mix the powdered sugar with the remaining 1 tablespoon limoncello and a few drops lemon juice until the mixture turns into a glaze. Add more lemon juice until the desired consistency is achieved. Drizzle the glaze over the top of the cake, letting some drip down the sides.