2Line an 8-inch square pan with a long sheet of foil that covers the bottom and reaches up the side, do the same for the other sides.
3Coat with non-stick cooking spray.
4In a medium saucepan, melt the butter, add the chocolate pieces and stir constantly over very low heat until the chocolate is melted.
5Remove from heat and whisk in the cocoa powder until smooth.
6Add the eggs one at a time, then stir in the sugar, vanilla, then the flour, add in the nuts, if using.
7Scrape half of the batter into the prepared pan.
8Drop 1/3 of the Dulce de Leche, evenly spaced over the brownie batter, then drag a butter knife through to swirl it slightly.
9Spread the remaining brownie batter over, then drop spoonfuls of the remaining Dulce de Leche in dollops over the top of the brownie batter and use the butter knife to swirl the Dulce de Leche slightly.
10Bake for 35 to 45 minutes. The brownies are done when the center feels just slightly firm.
11Remove from the oven to a wire rack to completely cool.
12These brownies taste even better the next day, and will keep well for up to 3 days.