Preheat the oven to 180 C / fan 160 C / Gas 4. Line the base and sides of a 20cm square loose-bottomed tin with baking paper.
Combine the butter and sugar in a large bowl. Add the vanilla extract and the eggs, one at a time, stirring each time.
Fold in the flour and white chocolate, then the raspberries until just combined. Be careful not to crush the raspberries too much. (If using frozen raspberries, work quickly to fold them in because they will start to harden the fat and that makes the mixture stiff.)
Spoon the mixture into the prepared tin and spread evenly. Bake for 1 hour. If the top gets too dark, cover with aluminium foil.
Remove from the oven and leave to cool in the tin. Once cooled, cut into squares.
Sprinkle 25g mini marshmallows on top after baking the brownies for 1 hour. Bake for 10 more minutes until their colour turns golden.