For the brownies, you'll need:

* 1 1/4 cup flour
* 2 tbsp cocoa powder
* pinch of salt
* 11 oz bittersweet chocolate
* 1 cup [2 sticks] unsalted butter, cut into cubes
* 1 cup granulated sugar
* 2/3 cup light brown sugar
* 5 large eggs, at room temperature
* 2 tsp vanilla

For the filling, you'll need

* 1 14-oz can dulce de leche
* 1 tsp salt
* 1 tsp fleur de sel, to top