Preheat oven to 350 degrees and line a 9x9 (or 8x8) pan with parchment paper OR greased foil and set aside.

In a large microwave-safe bowl, place butter, both sugars and cocoa powder. Microwave for 1 minute and 30 seconds. Stir very well and allow to cool for about 3 minutes before continuing. Add the eggs and vanilla and stir until incorporated. Spread the mixture into prepared pan and spread a layer of dulce de leche on top, swirl with a knife lightly - the batter is thick so it's hard to swirl. Top with remaining batter and swirl again slightly.

Bake brownies for 25-27 minutes or until a toothpick inserted in the center comes out clean. Allow brownies to cool completely in pan before cutting into bars. You may even want to refrigerate them for ease and then let them come back up to room temperature before serving.