Step 2

Make the Dulce de Leche: Preheat the oven to 425°F. Pour the sweetened condensed milk into a small, heatproof baking dish or pie plate. Place the baking dish inside another, larger baking dish and fill with enough hot water to come halfway up the dish. Cover tightly with aluminum foil and bake until deeply golden brown, 1 hour to 1 hour 15 minutes.

Step 3

Remove the dulce de leche from the oven and whisk while still hot, then set aside to cool while you make the brownie. (The dulce de leche can be made up to 5 days in advance and stored in an airtight container in the fridge.)

Step 4

Make the Brownie: Preheat the oven to 325°F. Lightly spray a 10-inch cast iron skillet with nonstick spray. In a small, microwave-safe bowl, combine the butter, chocolate and oil. Microwave in 30-second increments, stirring between each, until fully melted. Set aside. 4.In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt. Set aside.

Step 5

In a large bowl, whisk together the eggs, egg yolk, vanilla, brown sugar and granulated sugar until fully combined. Slowly whisk the chocolate mixture into the egg mixture, then fold in the flour mixture until just combined.

Step 6

Pour the batter into the prepared skillet. Use a spoon to dollop some of the dulce de leche all over the surface, then use a paring knife to swirl it into the batter. You may not use all the dulce de leche, and that's OK. Store it in an airtight container in the fridge for up to 5 days.

Step 7

Bake until the edges are crisp and the middle is crackly and looks set, 30 to 40 minutes (if you like a gooey brownie, err on the underbaked side). Cool slightly before serving with unsweetened whipped cream or vanilla ice cream, if desired.