Preheat the oven to 350°F (175°C). Line an 8x8-inch pan with parchment paper, leaving overhang for easy lifting. Lightly grease the parchment.

Melt butter in a light-colored saucepan over medium heat, stirring until it foams, turns amber, and smells nutty, about 5-7 minutes. Remove from heat.

Whisk in cocoa powder and espresso into the hot butter to bloom the flavors. Set aside.

Add chopped chocolate to the warm butter mixture and stir until smooth. If necessary, place over low heat in 10-15 second bursts to finish melting. Cool for 5 minutes.

In a large bowl, whisk together granulated sugar, brown sugar, eggs, and yolk for 1-2 minutes until thick and slightly lightened.

Whisk in vanilla, then add the chocolate-butter mixture, stirring until glossy and unified.

Sift flour and salt over the mixture and fold gently until just combined. If using chocolate chunks, fold them in now.

Spread the batter evenly in the prepared pan. Tap the pan once to settle. Sprinkle flaky salt on top if desired.

Bake for 24-28 minutes, until the edges are set and a toothpick inserted 2 inches from the edge comes out with moist crumbs.

Cool in the pan on a rack for at least 1 hour. Lift out and slice with a hot, wiped knife for clean edges.