Preheat the oven to 350 degrees.

Line a 8-inch square baking dish with a parchment paper or spray pan with non stick cooking spray.

Add butter and semi-sweet chocolate to a microwave safe bowl.

Microwave in 30 second intervals, stirring until the chocolate is melted.

Whisk in the cocoa powder until smooth. Add in eggs one at a time, then stir in the sugar, vanilla, then the flour.

Pour half of the batter into baking pan. 

Spoon one-third of the Dulce de Leche, over the brownie batter evenly in dollops, then drag a knife or spatula through and swirl.

Pour the remaining brownie batter over, then spoon the remaining Dulce de Leche in dollops over the top of the brownie batter.

Use a knife or spatula to swirl the Dulce de Leche throughout the brownie batter.

Bake for 35 to 45 minutes.

The brownies center will be slightly firm when they are done. Remove from the oven and allow cool completely.