Turn oven on, set to 350°F (180°C).
Spray 9" x 13" pan with oil or line with parchment.
In a double boiler (or a metal bowl over a pan of hot water) melt and mix together chocolate, butter, and cocoa powder until smooth. Remove from heat and let cool for about five minutes.
In a large bowl whisk together eggs, brown sugar, and vanilla until light and creamy.
Whisk in the chocolate mixture until fully combined.
Using a spatula, fold in the flour just until until combined.
Pour into prepared pan and even out.
Drop spoonfuls of the Dulce De Leche onto the top and using either a paring knife or a skewer -- we used a chopstick -- go through the Dulce De Leche to create a marbled effect.
Bake until the brownies are dry and set on top but still gooey and fudgy inside, 25-40 minutes.
Cool in the pan on a wire rack. Cut into squares.