In a medium bowl, mix graham cracker crumbs, melted unsalted butter, and granulated sugar until combined. Press firmly into the bottom of a 9-inch springform pan. Chill in the refrigerator for 15 minutes to set.

In a large bowl, beat softened mascarpone cheese with powdered sugar until smooth. Add the zest and juice of the lemons and mix until combined.

In a separate bowl, whip cold heavy cream to medium-stiff peaks. Gradually fold the whipped cream into the mascarpone mixture gently. If using, fold in Limoncello liqueur.

Spread the mascarpone mixture evenly over the chilled crust, smoothing the top. Cover with plastic wrap and refrigerate for at least 4 hours or overnight.

Just before serving, whip cold heavy cream with powdered sugar until stiff peaks form. Spread or pipe over the chilled cake.

Decorate with lemon slices or extra lemon zest. Slice and serve cold.