Preheat the oven to 350°F. Spray a 13-by-9-inch baking pan with nonstick spray, line with parchment letting it overhang, and lightly coat again.
STEP 2
Melt the butter in a saucepan over medium heat until golden brown, about 5 minutes. Add chopped chocolate and cream; stir until melted.
STEP 3
Sift flour and cocoa powder together. In a stand mixer, beat granulated sugar, brown sugar, salt, vanilla, espresso powder, and eggs until pale and thick, about 8 minutes. Reduce speed and gradually add warm chocolate. Slowly add dry ingredients; mix until just combined.
STEP 4
Scrape batter into prepared pan. Bake until glossy and firm, about 25 to 35 minutes, checking with a thermometer for 205°F in the center.
STEP 5
Cool completely for around 3 hours before slicing into 24 squares.