| 2 1/2 sticks butter, at room temperature |
| 1 c. sugar |
| 140 g. packed light brown sugar (1/2 c. + 2 t.) |
| 60 g. corn syrup (1/4 c.) |
| 1/2 t. vanilla extract |
| 4 large eggs, at room temperature |
| 125 g. AP flour (3/4 c. + 2 T.) |
| 125 g. Dutch processed cocoa powder (1 c.) |
| 1/4 t. salt |
| 1 c. dulce de leche (or more, if desired) |
| flaky sea salt |
Adjust oven rack to middle position and preheat oven to 350°F. Butter a 10x6" brownie pan and line the bottom and sides of the pan with parchment paper.
Using an electric hand mixer, beat butter until creamy and lightened in color, about 1 minute. Add granulated and brown sugars, corn syrup, and vanilla and mix until incorporated. At eggs one at a time, mixing after each addition until incorporated.
In a small bowl, stir together flour, cocoa powder, and table salt. Add dry ingredients to butter mixture, and, using a rubber spatula, fold until mostly incorporated. Beat batter with electric mixer until smooth, about 30 seconds.
Pour batter into the prepared pan. Dollop spoonfuls of dulce de leche onto the batter, using a knife to swirl gently to partially mix the two together (be careful to keep the dulce de leche on top as you swirl).
Bake for 15 minutes, then sprinkle generously with sea salt. Return to oven and bake until the edge is set but the center is still a bit soft (about 20 minutes, but see note below). Let the brownies cool to room temperature in the pan, then cut and serve.