Preheat the oven to 350°F (177°C). Line an 8-inch square baking pan with parchment paper, leaving some overhang on the sides for easy removal later.

Measure out ½ cup (138 g) of dulce de leche and set aside for swirling in later.

Add chocolate chips and remaining dulce de leche to a large microwave-safe bowl. Heat in the microwave at full power in 15-30 second increments, stirring in between with a spatula, until the chocolate is completely smooth and melted. The mixture will also be very thick.

Let the chocolate mixture cool a little so it is still warm but no longer too hot to touch. Then add in the egg. Stir until the egg is fully incorporated and no egg streaks remain. Adding the egg should also make the batter thinner.

Transfer the batter to your prepared baking pan. Add dollops of remaining dulce de leche on top of the batter. Use a butter knife to swirl the dulce de leche into the batter.

Bake the brownies for about 30 minutes. During the first part of baking the brownie batter will look unchanged but towards the end of the baking time you should see the brownie batter rise a little. Your brownies are done when the batter has puffed up and a toothpick inserted has only a few crumbs attached.

Allow brownies to cool fully before cutting with a sharp knife. Because the brownies are so fudgy, I like to heat the blade with warm water before cutting and wipe the blade clean after each cut.