Preheat and line: Set the oven to 180C (160C fan / 350F / Gas 4). Butter a 23cm (9 inch) square cake tin and line with greaseproof paper, leaving overhang on two sides for easy lifting.

Melt and whisk the base: Melt the butter in a saucepan over a low heat. Remove from the heat and whisk in the dark brown sugar and salt. Give it a proper whisk here because this is what makes the mixture lighter and slightly fluffy. Add the vanilla extract and whisk again.

Add the eggs: Stir the beaten eggs into the butter mixture until smooth. You are looking for a rich, glossy paste.

Fold in the flour: Sift the plain flour, bicarbonate of soda, and baking powder into a large bowl. Pour the wet mixture in a little at a time, whisking between additions. Adding the flour in stages stops it going lumpy. It should drop off the whisk cleanly.

Add chocolate and cranberries: Let the batter cool for a couple of minutes, then fold in the white chocolate chunks and dried cranberries. Do not overmix. If the batter is still hot, the chocolate melts and you lose those gooey pockets.

Bake: Spoon into the prepared tin and smooth the top with a spatula, working into the corners. Bake for 35 to 40 minutes until the outer edges are firm and the middle is still slightly soft.

Cool and cut: Leave in the tin on a wire rack for at least 10 minutes before cutting into 9 squares. The centre firms up as it cools, so do not panic if it looks wobbly when it comes out.