Preheat the oven to 350ºF (180ºC).
Butter or spray an 8-inch (20 cm) square baking pan. Line it with parchment paper covering, see Notes below.
Melt ½ cup unsalted butter and 6 ounces semisweet chocolate in a bowl over simmering water, stirring until melted and combined. Or use the microwave in 15-second intervals, mixing well after each. Be careful not to burn or char the chocolate.
Remove from heat and add 3 eggs, mixing well with a whisk.
Add ½ cup white sugar, ½ cup brown sugar and ½ teaspoon vanilla extract and integrate them well.
Sift in 1 cup all-purpose flour, ¼ cup unsweetened cocoa powder and pinch of salt. Stir to mix well.
Pour the batter into the prepared pan.
Drop 1 cup regular dulce de leche by spoonfuls, making 9 mounds. With the tip of a knife, swirl both batters slightly. Don't overmix them; otherwise, you won't get the puddles of dulce de leche we want.
Bake for 25-30 minutes. The brownies are done when the center still jiggles a little. Do not overbake. Remove from the oven and cool for 10 minutes.
Run a smooth-bladed knife around the edges to loosen any bits of dulce de leche or batter that might've stuck. Cool completely on a wire rack.
These brownies are better the next day and will keep well for up to 3 days, covered in plastic wrap or in an airtight container.