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Tip most of the raspberries into a saucepan (reserving a handful), along with 1 tsp brown sugar and 2 tsp water. Cook over a medium heat until the raspberries have broken down and turned jammy. Remove from the heat and set aside to cool while you make the batter. ½
Melt the butter in a second pan over a low-medium heat. Tip in both the sugars and cook until the mixture is bubbling and the sugars have dissolved. Remove from the heat and leave to cool for 10 mins.
Meanwhile, combine the flour, baking powder and a good pinch of salt in a large bowl. Butter a 20 x 30cm tin and line with baking parchment. Heat the oven to 180C/160C fan/gas 4.
Pour the wet ingredients into the dry, add the eggs and vanilla, and mix well until there are no pockets of flour. Scatter over the dark and white chocolate chunks and fold to combine. Scrape the mixture into the prepared tin and level the surface with a spatula. Dot over teaspoonfuls of the raspberry compote, then use a skewer to gently swirl it through the batter to create a marbled effect. Scatter over the reserved raspberries, then bake for 40-45 mins until set with a slightly gooey centre. Leave to cool completely in the tin, then cut into squares to serve. Will keep for up to three days in an airtight container.