Preheat oven to 350°F. Line an 8-inch square baking dish with parchment paper and spray with nonstick cooking spray.

In a medium saucepan, melt the butter over medium-low heat. As the butter melts, it’ll start to foam, then turn a clear golden color. Continue to stir the butter, scraping up any browned specks from the bottom of the pan as it starts to brown. It should have a nutty aroma.

Once browned and nutty, turn off the heat. Working relatively quickly so the butter doesn't continue browning from the residual heat in the pan, whisk in chopped chocolate until melted and smooth. Set aside to cool slightly.

Add both sugars to the butter mixture and whisk vigorously until combined. Allow to cool until just barely warm.

Add the eggs, yolk, and vanilla, and whisk until smooth.

Using a rubber spatula, fold in the flour, cocoa powder, salt and espresso powder until just combined. Stir in the walnuts.

Pour half of the batter into the baking pan. Dollop half of the dulce de leche evenly on top, then swirl with a knife. Pour the rest of the batter on top, followed by dollops of the rest of the dulce de leche, and swirl once again with a knife.*