Prep the pan: Heat oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving a little overhang for easy lifting. Lightly grease the parchment.

Toast the walnuts: Spread walnuts on a sheet pan and toast for 6–8 minutes until fragrant. Cool, then chop. This step boosts flavor and keeps them crunchy.

Melt butter and chocolate: In a heatproof bowl set over a pot of barely simmering water, melt the butter and dark chocolate, stirring until smooth. Remove from heat and cool 5 minutes. You can also use the microwave in 20–30 second bursts, stirring between each.

Whisk in sugars: Add granulated and brown sugars to the warm chocolate mixture. Whisk until glossy and combined. The warmth helps dissolve the sugar a bit and gives you that shiny top later.

Add eggs and vanilla: Whisk in eggs one at a time, fully blending after each. Stir in vanilla. Beat for about 30 seconds until the batter looks thick and satiny.

Combine dry ingredients: In a small bowl, whisk flour, cocoa powder, kosher salt, and espresso powder (if using). This prevents cocoa clumps.

Fold to finish the batter: Using a spatula, fold the dry ingredients into the wet just until you see no dry streaks. Do not overmix. Fold in most of the chopped walnuts, saving a handful for the top.

Pan and top: Spread batter evenly in the prepared pan. Sprinkle remaining walnuts on top. If you like, add a small pinch of flaky salt.

Bake: Bake 24–28 minutes. Start checking at 24. The edges should be set and slightly pulled from the pan; a toothpick in the center should come out with moist crumbs, not wet batter.

Cool and set: Let brownies cool in the pan on a rack for at least 1 hour. For the cleanest slices and peak fudginess, chill 30 minutes after they reach room temperature, then lift and cut.

Slice smart: Use a sharp knife, wiping between cuts. Aim for 9 large squares or 16 smaller ones.