Preheat your oven to 350°F (175°C). Grease your 8x8-inch baking pan or line it with parchment paper for easy removal.
In a medium saucepan over low heat, combine the chopped chocolate and butter. Stir until melted and smooth. Remove from heat and let cool slightly.
In a separate bowl, whisk together the cocoa powder and flour. Set aside.
In a large mixing bowl, whisk the eggs, sugar, and vanilla extract until well combined and slightly frothy.
Gradually pour the melted chocolate mixture into the egg mixture, stirring continuously until fully incorporated.
Fold the dry ingredients into the wet mixture using a rubber spatula. Be careful not to overmix; just combine until no flour streaks remain.
If using, fold in the toasted pecans or walnuts until evenly distributed throughout the batter.
Pour half of the brownie batter into the prepared pan. Then, dollop half of the dulce de leche on top. Use a knife to swirl it gently into the batter. Repeat with the remaining batter and dulce de leche.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the brownies to cool in the pan for at least 15 minutes before lifting them out using the parchment paper. Cut into squares and enjoy your Dulce De Leche Brownies!
