Preheat oven to 180 degrees celsius (fan-forced). Grease and line a 20cm X 20cm square cake tin with baking paper.
Place the butter into the Thermomix bowl and melt for 3 minutes, 80 degrees, Speed 2. Add the brown sugar and mix on Speed 5, 10 seconds. Scrape down the sides of the bowl and repeat for a further 10 seconds or until smooth and creamy.
Add the egg and vanilla extract and mix on Speed 4, 15 seconds. Scrape down the sides of the bowl.
Add the plain flour and salt and mix on Speed 4, 5 seconds. Scrape down the sides of the bowl and mix for a further 5 seconds.
Fold through half of the white chocolate chips and half of the raspberries with the spatula. Spoon the batter into the cake tin and spread out with a spoon.
Sprinkle over the remaining raspberries and white chocolate chips.
Bake for 30 minutes or until a toothpick inserted into the middle comes out with a few crumbs.
Allow to cool before transferring to a wire rack to cool completely. Store in an airtight container in the fridge for up to 3 days.