Prep the pan: Heat the oven to 350°F (175°C). Line an 8-inch (20 cm) square pan with parchment, leaving an overhang for easy lifting.Lightly grease the parchment.
Melt butter and chocolate: In a heatproof bowl, melt the butter and chocolate together over a saucepan of simmering water, stirring until smooth. You can also use a microwave in 20–30 second bursts. Let it cool for 3–4 minutes.
Add sugars: Whisk in the granulated and brown sugars until glossy and well combined.
Whisk in eggs: Add the eggs and yolk one at a time, whisking well after each.Whisk for about 30–45 seconds until the mixture thickens slightly and looks shiny. Stir in the vanilla.
Combine dry ingredients: In a separate bowl, whisk flour, cocoa powder, salt, and espresso powder. Sift if lumpy.
Fold the batter: Use a spatula to fold the dry ingredients into the wet until just combined.Stop as soon as no dry streaks remain. Overmixing leads to cakey brownies.
Spread and swirl: Pour the batter into the pan and smooth the top. Warm the cookie butter for 10–15 seconds so it’s pourable.Dollop spoonfuls over the batter and use a butter knife to gently swirl. Don’t over-swirl—3–4 passes are enough.
Add crunch: Sprinkle the crushed Biscoff cookies evenly over the top. Press them in lightly so they stick.
Bake: Bake for 24–28 minutes, until the edges are set and the center has a slight jiggle.A toothpick should come out with moist crumbs, not wet batter. Err on the side of underbaked for fudgy texture.
Cool and set: Cool the pan on a rack for at least 1 hour before slicing. For the cleanest squares, chill for 30 minutes after cooling, then cut with a warm knife.
Finish: Sprinkle with flaky sea salt if using.Lift out using the parchment and slice into 9 large or 16 smaller squares.
