Have the can of dulce de leche ready with you at room temperature.
Cut the butter in cubes and roughly chop the chocolate. Then melt the butter, chocolate and cocoa powder over a double boiler. Remove from heat and keep aside when the chocolate and the butter have incorporated in a nice smooth liquid.
In another mixing bowl, whisk the eggs with sugar and vanilla extract. Whisk until its light and frothy, approximately for 2 minutes using an electric hand mixer.
While continuing to whisk, slowly pour the liquid chocolate from one corner of the bowl and combine.
Sift in the flour and gently fold with a spatula.
Have a 20 cm or 8 inch square cake tin or a rectangular brownie pan ready with you. Instead of simply greasing it, I always prefer sticking butter paper to the bottom of the cake tin and greasing the sides.
Pour the brownie mix into the tin and spread using a spatula. Then use a spoon to scoop out dulce de leche from the can and drop blobs of dulce de leche all ovr the cake. Using a skewer or a knife, run it around to make swirls and combine the dulce de leche into the brownie. It is a marbeled effect.
Then transfer the cake tin into the pre heated oven and bake at 180 degrees C for about 20 to 25 minutes. In the case of these brownies, your skewer will not come out clean once you stick it in. The idea is to have a gooey or a fudgy center.
In 20 to 25 minutes, the top and the bottom should be done. Take it out and leave it on a wire rack to cool down.
Once cool, cut it into squares and serve them.