In a medium bowl, combine graham cracker crumbs and a pinch of salt. Stir in melted butter until the mixture resembles damp sand. Press firmly into a 9-inch pie plate, building the crust up the sides about 1/4 inch. Chill in the refrigerator while you make the filling.

In a chilled bowl, beat the heavy cream with an electric mixer until soft peaks form. Add vanilla if using and beat just until soft-stiff peaks.

In a separate bowl, whisk the sweetened condensed milk with lemon juice and lemon zest. Adjust tartness by adding more lemon juice if desired.

Gently fold half of the whipped cream into the lemon-condensed milk mixture to lighten it, then fold in the remaining whipped cream until streak-free and smooth.

Spoon the filling into the chilled crust and smooth the top with an offset spatula. Cover loosely with plastic wrap and refrigerate for at least 4 hours, or overnight for a firmer slice.

When ready, zest a little extra lemon over the top and add dollops of whipped cream if desired. Slice with a sharp knife dipped in hot water for clean edges.