Preheat oven to 180°C (350°F).

Prepare a 10x7 inch brownie pan or a 9 inch square pan by greasing or lining with parchment paper.

Sift flour, cocoa powder, and baking powder in a bowl and set aside.

Place butter and chopped dark chocolate in a microwave-safe bowl.

Microwave for about a minute, or until melted, or use a double boiler.

Stir the melted butter and chocolate until smooth and let it cool slightly.

In another bowl, whisk eggs with a hand mixer until fluffy.

Add the melted chocolate mixture to the eggs and beat to combine.

Gradually add sugar and vanilla extract and beat until well blended.

Fold in the sifted flour mixture and walnuts or pecans until just combined.

Pour the batter into the prepared pan and spread evenly.

Drop spoonfuls of dulce de leche at intervals on top of the batter.

Use a fork or knife to gently swirl the dulce de leche on top.

Bake in the preheated oven for 35-40 minutes, or until a tester inserted in the center comes out clean.

Remove from oven and let cool completely before serving.

Serve as a snack or with vanilla ice cream for dessert.