Prep your pan. Grease and line an 8x8-inch baking pan with parchment paper. Leave a bit of overhang for easy removal.
Make the crust. Beat the butter and sugar together in the bowl of a stand mixer or with a hand mixer. Beat for about 5 minutes, until the butter and sugar are very fluffy. Add the flour and vanilla and beat until just mixed.
Chill the crust. Press into an even layer in the prepared pan and chill 1 hour.
Bake the crust. Preheat an oven to 350ºF. Prick the crust all over with a fork. Bake for 20 minutes, until golden brown and slightly puffed. Cool to room temperature.
Make the filling. Lower the oven temperature to 325ºF. In a mixing bowl, whisk together the sweetened condensed milk, eggs, vanilla, brown sugar, and salt until smooth. Stir in the chopped pecans. Pour the filling over the cooled crust.
Bake at 325ºF. Bake for 50-55 minutes, until the edges are puffed but the middle is still slightly jiggly. Remove from the oven and sprinkle the top with flaky sea salt, if desired.
Cool, then chill. Cool the bars for 1 hour at room temperature and then refrigerate for 2 hours before cutting and serving.