Melt the butter and chopped chocolate together in a heatproof bowl over barely simmering water, stirring until smooth. Remove from heat and let cool slightly.

With an electric mixer, beat the eggs, granulated sugar, brown sugar, vanilla, and espresso powder on medium-high until thick, pale, and ribbony, about 3 minutes.

Pour the cooled chocolate mixture into the egg mixture and fold gently with a spatula until combined.

Sift the flour, cocoa powder, and salt into the bowl and fold until just combined, then fold in the chocolate chunks.

Preheat the oven to 350°F (175°C) and line an 8-inch square metal pan with parchment. Scrape the batter in, smooth the top, and bake for 30 to 35 minutes, until a toothpick comes out with moist crumbs.

Cool completely in the pan, then lift out and slice into squares.