Preheat the oven to 350º F.

Add the butter and chocolate to a medium heat proof bowl and set over a small pot of simmering water.  Stir the chocolate and butter, as needed, until melted, about 5 minutes.  Remove from the heat.

Add the cocoa powder to the melted chocolate and butter and whisk to combine.  Stir in the eggs, vanilla, sugar and flour.

Add the dulce de leche to a piping bag with a wide tip attached.  Set aside.  Grease a 9- by 13-inch baking pan or a 9 1/2- by 13-inch rimmed baking sheet.  Line with parchment and grease again.

Pour half of the brownie batter in the prepared pan.  It will be thick.  You can use an offset spatula or rubber spatula to spread it around.  Pipe half of the dulce de leche in lines across the batter then top with the remaining batter.  Pipe the remaining dulce de leche on top.  Using a toothpick or the tip of a knife, swirl it decoratively.

Bake for 40 to 45 minutes, or until set.  Set aside to cool. 

Once the brownies are completely cool, invert the pan onto a cutting board to remove the brownies.  Remove the parchment and discard.  Cut the brownies into squares, turn them so that they are facing right side up, and serve.

*Note:  You can make your own dulce de leche.  Set an unopened 13.4 ounce can of sweetened condensed milk in a large pot and cover with water.  Simmer for three hours, adding water as necessary, so that as the water is boiling away the can is always covered.  Remove the can from the pot and set aside until cooled to room temperature.  Open with a can opener and use as needed.