Combine graham cracker crumbs, melted butter, and sugar. Press into springform pan and chill.
Beat mascarpone, powdered sugar, lemon zest, and juice until smooth. Add heavy cream and whisk until fluffy. Fold in limoncello.
Spread filling over crust, cover, and chill 4 hours or overnight.
Whip topping cream and powdered sugar until stiff. Spread on cake.
Garnish with lemon slices or zest before serving.
