Preheat the oven to 350° F.

Line a 8-inch (20 cm) square pan with a long sheet of parchment paper to cover the bottom and all sides. use 2 pieces of paper if need be to cover all sides.

Melt the butter in a medium saucepan. Add the chocolate pieces and stir constantly over very low heat until the chocolate is melted. Remove from heat and whisk in the cocoa powder until smooth. Add in the eggs one at a time, then stir in the sugar, vanilla, then the flour.

Scrape half of the batter into the prepared pan leaving lots of big peaks and valleys.

Drop 1/3 of the Dulce de Leche, evenly spaced, over the brownie batter, then drag a knife through to swirl it (very) slightly (I completely over swirled mine). Spread the remaining brownie batter over, then drop spoonfuls of the remaining Dulce de Leche in dollops over the top of the brownie batter. Use a knife to swirl the Dulce de Leche very slightly.

Bake for 35 to 45 minutes, until a toothpick or knife comes out clean from the brownie batter. Remove from the oven and cool completely before cutting into 12-16 brownies.