Preheat oven to 350°F (175°C). Spray a 9×13" baking pan with nonstick cooking spray. Line with parchment paper with an overhang on the sides and spray again.

In a large mixing bowl, whisk together melted butter, granulated sugar, and brown sugar until sugar is dissolved.

Add the eggs and vanilla extract, whisk until smooth and combined.

Sift in flour and cocoa. Add salt and espresso powder, if using, and stir until just combined. Don't over mix.

Fold in 1 cup of chopped chocolate. Spread evenly into the prepared pan and sprinkle the remaining ¼ cup of chopped chocolate on top.

Bake for 25-30 minutes on the middle rack until the brownies are just set to touch. Don't over bake.

Remove from oven and let cool before cutting into squares. Sprinkle with flaky sea salt if desired.