Prep the pan: Heat the oven to 350°F (175°C). Butter a 9-inch square baking pan and line with parchment, leaving overhang on two sides for easy lifting.

Melt and whisk the base: Melt the butter in a saucepan over low heat. Off the heat, whisk in the dark brown sugar and salt until slightly lighter and fluffy, then whisk in the vanilla.

Add the eggs: Stir in the lightly beaten eggs until the mixture is a rich, glossy paste.

Fold in the flour in stages: Sift the flour, baking soda and baking powder into a large bowl. Pour the wet mixture in a little at a time, whisking between additions, until the batter is just dropping off the whisk. Do not overmix.

Cool, then fold in chocolate and cranberries: Let the batter sit 5 minutes until no longer hot, then fold in the chopped white chocolate and dried cranberries.

Bake: Spread into the prepared pan and bake 35 to 40 minutes, until the edges are firm and golden and the center is still slightly soft.

Cool and cut: Rest at least 10 minutes in the pan — the center firms up as it cools — then lift out and cut into 9 squares.