Line a 23cm square tin with baking paper leaving it hanging over the eges on two sides. Place condensed milk, butter and dark chocolate in a large bowl. Microwave on high for 1 minute, stirring frequently till melted. Whisk until smooth. Stir in the brazil nuts, blueberries and marshmallows. Pour mixture into the lined tin and spread out evenly.

Melt the white chocolate in the microwave for a minute on high. Give it a quick stir and drizzle over the mixture in the tin. Chill for 1-2 hours until set. Use the overhanging edges of the baking paper to lift the set mixture out of the tin. Cut into neat squares.