Preheat oven: Set to 350°F (180°C). Line an 8×8‑inch square pan with parchment paper.

Brown butter: In a stainless steel pan, melt cold butter over medium heat for 10–12 minutes. Stir as it foams and crackles until milk solids at the bottom turn golden brown and give off a nutty aroma. Immediately pour into a bowl to stop cooking.

Melt chocolate: Add chocolate chips to the hot browned butter and stir until melted and smooth. Set aside to cool slightly.

Mix dry ingredients: In a bowl, whisk together flour, cocoa powder, espresso powder, and salt.

Whip eggs and sugar: In another bowl, whisk eggs, sugar, and vanilla on medium speed for 1–2 minutes until light and fluffy.

Combine: Stir the cooled chocolate mixture into the egg mixture until blended. Gently fold in the dry ingredients until just combined. Do not overmix.

Bake: Spread batter evenly in the prepared pan. Bake for 30–40 minutes, or until a toothpick inserted near the center comes out with moist crumbs.

Cool & slice: Let brownies cool completely in the pan on a wire rack. Cut into 16 squares with a sharp knife, cleaning between cuts.