Prepare your pan. Preheat your oven to 350°F. Line an 8×8-inch baking pan with parchment paper, leaving a little overhang on the sides to make removing the brownies easier.

Melt the butter and sugars. Place the butter, granulated sugar, and brown sugar in a large microwave-safe bowl. Microwave until the butter is melted. Remove from the microwave and whisk for approximately 45 seconds to begin dissolving the sugars and slightly cooling the mixture.

Bloom the cocoa. Add the unsweetened cocoa powder and continue whisking for another 90 seconds. This helps deepen the chocolate flavor and encourages that lovely crinkly brownie top.

Add the eggs and vanilla. Add the eggs one at a time, whisking well after each addition until completely incorporated. Stir in the vanilla extract.

Combine the dry ingredients. In a separate bowl, whisk together the kosher salt, baking powder, and all-purpose flour.

Make the brownie batter. Add the dry ingredients to the chocolate mixture. Using a wooden spoon, gently stir just until no streaks of flour remain. Don't overmix! We want fudgy brownies, not chocolate cake.

Create the first layer. Spread half of the brownie batter evenly into your prepared pan. Dollop ¼ cup of dulce de leche over the batter. Using a butter knife, gently swirl it through the chocolate, being careful not to completely combine the two.

Finish the swirls. Carefully spread the remaining brownie batter over the first layer. Dollop the remaining ¼ cup of dulce de leche over the top and gently swirl again.

Bake. Bake for 30–35 minutes, or until a toothpick inserted into a brownie portion near the center comes out with a few moist crumbs. Avoid testing directly through a pocket of dulce de leche.

Cool and serve. Remove the brownies from the oven and allow them to cool in the pan for at least 2 hours before slicing. Sprinkle with flaky sea salt, if desired, then cut into squares and enjoy!