Preheat the oven to 350°F (175°C). In a mixing bowl, combine graham cracker crumbs, sugar, and melted butter. Mix until well combined.

Press the mixture firmly into the bottom of a greased 9x9 inch baking pan. Bake for 10 minutes, then remove and let cool.

In a separate mixing bowl, whip the heavy cream until soft peaks form.

In another bowl, combine mascarpone cheese, limoncello, powdered sugar, lemon zest, and vanilla extract. Mix until smooth.

Gently fold the whipped cream into the mascarpone mixture until fully combined.

Spread the filling over the cooled crust in the baking pan. Smooth the top with a spatula.

Cover and refrigerate for at least 4 hours or until set.

Cut into squares and serve topped with whipped cream and a sprinkle of lemon zest.