Preheat your oven to 350°F. Prepare a 9×13 inch baking pan by greasing it, lining it with parchment paper with a slight overhang, and then lightly greasing the parchment. This ensures the brownies lift out easily once cooled.

In a large mixing bowl, whisk together the melted butter, granulated sugar, and dark brown sugar until the mixture looks glossy and the sugars begin to dissolve. Add the eggs and vanilla extract, whisking until the batter becomes smooth and fully combined.

Sift the flour and cocoa powder directly into the bowl, then add the salt and optional espresso powder. Stir gently until the dry ingredients are just incorporated. Avoid overmixing, as this can affect the fudgy texture. Fold in 1 cup of the chopped chocolate.

Spread the batter evenly into the prepared pan and sprinkle the remaining 1/4 cup of chopped chocolate over the top for extra richness. Bake for 25 to 30 minutes, or until the top is set and the center is slightly soft but no longer jiggly.

Let the brownies cool completely in the pan before lifting them out using the parchment overhang. Once cooled, slice into squares and, if desired, finish with a light sprinkle of flaky sea salt.