Preheat oven to 350°F. Grease a 9-inch springform pan with butter.
Beat the softened butter, sugar, and salt with a hand mixer until creamy. Add the eggs one at a time to the butter-sugar mixture, beating well after each addition.
Zest and juice the organic lemon. Stir the lemon zest and 1-2 tablespoons of lemon juice into the batter.
Sift together the flour, cornstarch, and baking powder. Add this mixture alternately with the Limoncello to the batter and mix well.
Pour the batter into the prepared springform pan and smooth the surface. Bake for about 35-40 minutes. Test for doneness with a toothpick before removing from the oven.
Let the cake cool slightly in the pan, then release the sides of the springform pan and let cool completely.
Mix the mascarpone with the powdered sugar and 1-2 tablespoons of lemon juice until smooth. Beat the heavy cream until stiff peaks form and gently fold it into the mascarpone mixture.
Spread the mascarpone cream evenly over the cooled cake. Dollop the lemon curd onto the mascarpone cream and spread gently to create an even layer.
Chill the cake in the refrigerator for about 1-2 hours to allow the flavors to meld.