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

Melt the base: In a heatproof bowl set over a pot of barely simmering water, melt 6 oz white chocolate with the butter. Stir gently until smooth. Remove from heat and let cool 5–7 minutes so it’s warm, not hot.

Whip eggs and sugar: In a mixing bowl, whisk granulated sugar, brown sugar, eggs, and egg yolk for 2–3 minutes until thick, pale, and slightly foamy. This helps create a crackly top.

Combine: Slowly whisk the melted chocolate-butter mixture into the egg mixture. Stir in vanilla.

Add dry ingredients: Sift flour and salt over the bowl. Fold gently with a spatula just until no dry spots remain. Do not overmix.

Stir in extras: Fold in the remaining 2 oz chopped white chocolate and any optional mix-ins. The batter will be thick and glossy.

Spread and smooth: Scrape batter into the prepared pan and smooth the top. Tap the pan lightly on the counter to release air bubbles.

Bake: Bake 22–28 minutes, until the edges are set and lightly golden and the center has a slight jiggle. A toothpick should come out with a few moist crumbs, not wet batter.

Cool completely: Place the pan on a rack and cool at least 1–2 hours. For clean slices and the fudgiest texture, chill for 30 minutes before cutting.

Slice and serve: Lift out using the parchment and cut into 12–16 squares. Wipe the knife between cuts for neat edges.