Preheat oven to 350 degrees. Line a 8-inch square baking dish with foil. Spray with non-stick cooking spray.

In a small saucepan melt the butter. Add the semi-sweet chocolate, stirring constantly over low heat until the chocolate is melted.

Remove from heat and whisk in the cocoa powder until smooth.

Add eggs one at a time, then stir in the sugar, vanilla, and salt.

Stir in the flour, then the nuts, if using.

Pour half of the batter into the prepared pan.

Dollop half of the Dulce de Leche over the brownie batter and drag a knife through the Dulce de Leche to swirl.

Pour the remaining brownie batter on top, then spoon on the remaining Dulce de Leche, swirling again with a knife to create a marbled effect.

Bake for 35 to 40 minutes. The brownie's center will be slightly firm when they are done. Remove from the oven and allow cool completely before cutting into bars.