Preheat oven to 350°F. Spray the inside of an 8-inch square baking pan with nonstick cooking spray or oil. Line the pan with parchment paper, then spray the parchment with nonstick cooking spray or oil.
Add flour, cocoa powder, salt, and cinnamon to a large bowl. Whisk to combine.
In a small saucepan over medium-low heat, add the butter and chocolate chips. Stir continuously until the mixture has completely melted.
Remove from heat. Add the granulated sugar, brown sugar, and vanilla extract. Stir until well combined.
Add the eggs and egg yolk. Stir until well combined.
Pour the wet ingredients into the flour mixture. Fold together with a rubber spatula or wooden spoon until just combined. Try not to overmix.
Measure out ½ cup of the brownie batter and set aside. Pour the remaining batter into the prepared pan.
Warm up the dulce de leche in a small bowl in the microwave for about 1 minute, stirring halfway through, so that it’s easier to work with and not as thick.
Pour the dulce de leche over the brownie batter, then spread it out evenly with the back of a spoon to get as much coverage as possible.
Dollop the reserved ½ cup of brownie batter over the dulce de leche. Gently swirl the mixture together by running a knife lengthwise and crosswise through the top layers. Try not to stick the knife all the way to the bottom of the pan.
Bake for 40 minutes, until the center is firm and a toothpick inserted into the center comes out mostly clean.
Let the brownies cool completely in the pan, then transfer them to a cutting board and slice them into 16 small squares.
Notes
Warm up the dulce de leche in the microwave or stove to make it easier to work with and more pourable. Dulce the leche thickens as it cools, so warming it up makes it easier to spread.
Let the brownies cool completely before cutting them into squares and eating. This is important because the batter needs time to fully set. If you slice them before they cool, they’ll be hard to work with and too fudgy and gooey.
Adding a sprinkle of flaky sea salt on top of the brownies when you take them out of the oven helps to balance all the rich and indulgent chocolate and dulce de leche flavors. A touch of salt and dulce de leche is a match made in heaven!