In a medium bowl, combine graham cracker crumbs, melted butter, and granulated sugar. Mix until well combined. Press the mixture firmly into the bottom and up the sides of a 9-inch pie plate.
Bake the crust for 8-10 minutes, or until lightly golden. Let cool completely.
While the crust cools, prepare the caramel filling. In a medium saucepan, combine sweetened condensed milk, cubed butter, and brown sugar. Cook over medium heat, stirring constantly, until the mixture thickens and turns a deep caramel color, about 10-15 minutes. Be careful not to burn it.
Remove from heat and stir in the vanilla extract and sea salt.
Pour the warm caramel filling into the cooled pie crust. Smooth the top with a spatula.
Chill the pie in the refrigerator for at least 4 hours, or until firm. This can also be done overnight.
Once chilled, top with whipped cream, drizzle with caramel sauce, and sprinkle with flaky sea salt before serving, if desired.
Notes
This pie is best served cold. For an extra rich flavor, you can toast the graham crackers before crushing them.