Remove the label from the can of sweetened condensed milk. Then, in a large sauce pot, add the can and cover with water, at least an inch over the top. Bring to a boil, then reduce to a simmer and simmer for 3 hours, rotating the can occasionally. Bring a small sauce pot of water to a simmer, to use for topping off the water in the large sauce pot.

Never allow the water to drop below the line of the can, continuously topping off with simmering water from the small sauce pan. After 3 hours, carefully remove the can from the water and allow to cool for several hours, or until the can is room temperature.

Preheat oven to 350°F and grease and line a 13×9 metal baking dish with parchment paper.

Add chocolate, cocoa, and espresso powder to a heatproof bowl. In a sauce pot over medium heat, or a microwave, melt butter until foam subsides, then pour over chocolate mixture. Stir until chocolate is melted and mixture is well combined. Set aside to cool slightly.

In a medium bowl, whisk together flour, salt, and baking powder, and set aside.

In a large bowl, or the bowl of a stand mixer fitted with a paddle attachment, whisk eggs and sugars to ribbon stage (until light, fluffy, and mixture holds a figure 8 on the surface of the batter for 1-2 seconds when the whisk is lifted), 5-7 minutes. Add the vanilla extract and then the chocolate mixture slowly, whisking constantly, until well combined. Add dry ingredients and use a large rubber spatula to fold ingredients until just combined.

Pour batter into prepared pan and spread into an even layer.

Scoop dulce de leche into a small bowl and whisk vigorously to soften. Dollop ¾ of the can over the brownies, then use a butter knife to swirl the mixtures together.

Bake for 40-45 minutes, or until edges begin to pull away from sides of the pan and the center is just set.

Sprinkle with flaky salt. Cut into squares and serve.