1. Prepare the Crust:

1. In a medium bowl, mix the crushed graham crackers, melted butter, and sugar (if using) until the mixture resembles wet sand.
2. Press the mixture firmly into the bottom and up the sides of a 9-inch pie dish. Use the back of a spoon or the bottom of a glass to pack it tightly.
3. Place the crust in the refrigerator to chill while you prepare the filling.

2. Make the Lemon Filling:

1. In a large bowl, whisk together the sweetened condensed milk, freshly squeezed lemon juice, and lemon zest. The mixture will thicken naturally as the lemon juice reacts with the milk.
2. In a separate bowl, whip the heavy cream until soft peaks form. Gently fold the whipped cream into the lemon mixture until fully combined. Add vanilla extract if desired.

3. Assemble the Pie:

1. Pour the lemon filling into the chilled crust, spreading it evenly with a spatula.
2. Cover the pie with plastic wrap and refrigerate for at least 4 hours, or until the filling is firm. For best results, chill overnight.

4. Garnish and Serve:

1. Before serving, garnish the pie with whipped cream, lemon slices, or a sprinkle of lemon zest.
2. Slice, serve, and enjoy the refreshing, creamy goodness!