Preheat the oven to 350°F (175°C). Line a 9 by 13-inch pan with parchment paper, leaving an overhang.
Mix biscuit crumbs, melted butter, and sugar. Press firmly into the pan and bake for 8 minutes.
Beat softened cream cheese and sugar until smooth. Add the eggs one at a time, then mix in vanilla and sour cream.
Pour the filling over the crust. Spoon dulce de leche over the top and swirl gently with a knife.
Bake for 35 to 42 minutes, until the edges are set and the center has a slight wobble.
Cool completely, then refrigerate for at least 4 hours. Cut into bars and garnish with flaky sea salt if desired.