Preheat your oven to 350°F and line a 9x13-inch pan with parchment paper.
Mix graham cracker crumbs, melted butter, and sugar until combined, then press firmly into the bottom of the pan.
Bake the crust for 10 minutes, then let it cool slightly.
In a bowl, whisk together sweetened condensed milk, lemon juice, lemon zest, egg yolks, and vanilla until smooth and creamy.
Pour the filling over the crust and spread evenly.
Bake for 15–18 minutes, until the center is just set but still slightly soft.
Let cool completely, then refrigerate for at least 3 hours so it firms up perfectly.
Whip the heavy cream with powdered sugar and vanilla until soft peaks form.
Spread or pipe the whipped cream over the chilled bars and finish with a sprinkle of lemon zest.
