In a large bowl, whisk together the flour and salt. Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs. Gradually add ice water, 1 tablespoon at a time, until the dough just comes together.

Form the dough into a disc, wrap it in plastic wrap, and refrigerate for at least 30 minutes.

Preheat oven to 375°F (190°C). On a lightly floured surface, roll out the dough to a 12-inch circle. Transfer to a 9-inch pie dish, trim the edges, and crimp.

Prick the bottom of the crust with a fork. Bake for 15-20 minutes, or until lightly golden. Let cool completely.

In a medium saucepan, whisk together the water, erythritol (or monk fruit sweetener), cornstarch, and salt.

Cook over medium heat, stirring constantly, until the mixture thickens and becomes clear. Remove from heat.

Stir in the lemon juice, lemon zest, butter, and vanilla extract until smooth. Gently whisk in the heavy cream.

Pour the filling into the cooled crust. Refrigerate for at least 3 hours, or until set.

Garnish with lemon slices or whipped cream, if desired.