Preheat oven to 170ºC (not fan forced). Grease and line base and sides of a lamington/slice pan (16cm x 26cm base) with non-stick baking paper.
Chop 200g chocolate and place in a medium ovenproof bowl. Add butter. Microwave, uncovered, on high in 1 minute bursts, stirring with a metal spoon, for 2-3 minutes on until smooth and chocolate has melted. Set aside to cool slightly.
Sift flour and cocoa into a large bowl. Stir in sugar. Whisk in eggs, vanilla and cooled chocolate mixture. Fold through two-thirds of the frozen Raspberries. Pour mixture into pan, top with remaining frozen Raspberries. Gently press. Finely chop remaining 100g chocolate and sprinkle over Raspberries. Bake for 50 minutes or until set. Cool completely in pan.
Once cold, transfer to an airtight container or cover with foil. Cut into slices and serve.