Prepare the Baking Pan: Preheat the oven to 350°F (175°C). Coat the inside of an 8-inch square baking pan with nonstick cooking spray or oil. Line the pan with parchment paper, then lightly grease the parchment as well.
Combine the Dry Ingredients: Add the all-purpose flour, unsweetened cocoa powder, fine salt, and ground cinnamon to a large mixing bowl. Whisk until the ingredients are evenly distributed and no large clumps remain.
Melt the Chocolate and Butter: Place the unsalted butter and semisweet chocolate chips in a small saucepan over medium-low heat. Stir continuously until both ingredients have melted completely and the mixture is smooth.
Mix in the Sugars and Vanilla: Remove the saucepan from the heat. Add the granulated sugar, dark brown sugar, and vanilla extract. Stir until everything is thoroughly combined.
Incorporate the Eggs: Add the two large eggs and one additional egg yolk to the chocolate mixture. Stir until the eggs are fully incorporated and the mixture is uniform.
Combine the Wet and Dry Mixtures: Pour the chocolate mixture into the bowl of dry ingredients. Using a rubber spatula or wooden spoon, gently fold everything together until just combined. Avoid overmixing to maintain the brownies' rich, tender texture.
Reserve Some Brownie Batter: Measure out ½ cup of the prepared batter and set it aside. Transfer the remaining batter to the prepared baking pan, spreading it across the bottom in an even layer.
Warm the Dulce de Leche: Place the dulce de leche in a small microwave-safe bowl. Microwave for approximately 1 minute, stirring halfway through, until it becomes warmer, softer, and easier to spread. Warming helps loosen its thick consistency.
Add the Dulce de Leche Layer: Pour the warmed dulce de leche over the brownie batter in the pan. Use the back of a spoon to spread it as evenly as possible across the surface.
Create the Chocolate Swirls: Drop small spoonfuls of the reserved ½ cup of brownie batter over the dulce de leche layer. Gently draw a knife lengthwise and then crosswise through the top layers to create a marbled pattern. Avoid pushing the knife all the way to the bottom of the pan. Bake the Brownies: Place the pan in the preheated 350°F (175°C) oven and bake for 40 minutes. The brownies are ready when the center feels firm and a toothpick inserted into the center comes out mostly clean.
Cool Completely: Remove the pan from the oven and allow the brownies to cool completely in the pan. This resting period helps the layers set, making the brownies easier to handle and cut.
Slice and Serve: Once fully cooled, transfer the brownies to a cutting board. Cut into 16 small squares and serve.
