📋 Instructions

1. 1Heat the oven and line the pan — Preheat the oven to 350°F (175°C) and line an 8-inch square pan with parchment, leaving an overhang on two sides. The overhang is your handle for lifting the slab out later.
2. 2Melt the butter — Melt the butter in a small saucepan over low heat or in a microwave in 20-second bursts, then pour it into a large bowl and let it cool for 5 minutes. Warm, not hot, butter keeps the eggs from scrambling.
3. 3Whisk in sugar and eggs — Whisk the brown sugar into the butter until smooth, then beat in the egg, yolk, and vanilla for about 30 seconds until glossy. The mixture should look thick and shiny, like wet sand that holds together.
4. 4Fold in the dry ingredients — Add the flour, baking powder, and salt and fold with a spatula just until no dry streaks remain, about 15 strokes. Stop the moment it comes together; a few lumps are fine.
5. 5Add the chocolate and berries — Fold in three-quarters of the white chocolate, then gently fold in the raspberries in two quick turns. Scrape the batter into the prepared pan and spread it evenly with the spatula.
6. 6Top and bake — Scatter the remaining white chocolate over the top, then bake at 350°F (175°C) for 32 to 36 minutes. The edges will be golden brown and pulling from the pan, the center will look just set, and a toothpick inserted two inches from the edge comes out with moist crumbs.
7. 7Cool completely — Set the pan on a wire rack and cool for at least 1 hour, until the pan feels barely warm to the touch. This rest is what sets the fudgy center.
8. 8Lift and slice — Use the parchment overhang to lift the slab onto a cutting board and cut into 16 squares with a long, sharp knife, wiping the blade between cuts. Serve at room temperature.