Preheat the oven to 347°F (175 °C) and cover the tray with baking paper. The original recipe uses a rectangular oven dish, but I used an oval one. Melt the butter, chocolate and cocoa together over low heat, while stirring. Leave to cool slightly.
Mix the eggs and sugar into a fluffy cream and gradually add the chocolate mixture to them. While stirring constantly add the vanilla, flour and mix until you get a thick cake batter. Finally add the walnuts. Pour the mixture into the oven dish and spread the top with a spoon of dulce de leche.
Using a knife, mix the milk caramel and cake mixture on the surface. Put the brownies in to bake for about 35 minutes, but do not overdo it, because the inside must remain moist.
To prepare the dulce de leche put 1 can (unopened) of condensed milk to boil on a low stove for 3-4 hours. Periodically, add water because the container must be covered at all times. Otherwise, the can may swell and the contents spoil. Allow it to cool completely in the water and open it once fully cooled.