The best fudgy brownies — intensely chocolatey with a crackly top, dense center, and rich flavor from browned butter, Dutch process cocoa, and dark chocolate. Chill before slicing for maximum fudginess.
Preheat the oven to 350°F / 175°C. Line an 8x8 or 9x9 baking pan with parchment paper, leaving overhang on the sides for easy removal. Grease lightly.
Brown the butter in a saucepan over medium heat, swirling occasionally, until it smells nutty and has golden-brown flecks. Remove from heat.
Whisk the milk powder (if using) and cocoa powder into the hot browned butter until smooth. Add the chopped dark chocolate and oil, then stir until the chocolate is fully melted and the mixture is glossy. Set aside to cool slightly.
In a large bowl, whisk together the eggs, egg yolk, granulated sugar, powdered sugar, brown sugar, and vanilla extract until well combined and slightly thickened.
Pour the chocolate mixture into the egg mixture and whisk until smooth and glossy.
Fold in the flour, salt, and espresso powder (if using) with a spatula until just combined — do not overmix. Fold in the chocolate chips.
Pour the batter into the prepared pan and spread evenly.
Bake until the top is set and crackly but the center is still slightly underdone — a toothpick should come out with moist crumbs, not wet batter, about 25–30 minutes.
Let cool completely in the pan, then refrigerate for at least 1–2 hours before slicing for the fudgiest texture.