Preheat oven to 160°C fan forced. Grease and line a 5cm deep, 18cm x 28cm (base) slab pan.
Combine chocolate and butter in a microwave-safe heatproof bowl. Microwave, uncovered in 1 minute bursts of High Power/100%, stirring every minute until smooth. Whisk in the cocoa powder while the mixture is warm. Stir in caster sugar then eggs. Sift flour and baking powder together then stir into brownie mixture. Spread brownie mixture over the base of the pan. Bake for 25 minutes until a skewer inserted in centre comes out clean.
For the marshmallow, combine 1/3 cup of water and sugar in a small saucepan over medium heat. Stir until sugar dissolves. Stir the gelatine into the remaining 1/3 cup of the water then add to the sugar mixture. Stir over medium heat until gelatin has dissolved. Remove from heat, allow to cool for 30 minutes.
Pour the syrup into bowl of mix master. Whisk on high speed for 4-6 minutes, until thick. Spread the marshmallow over the brownie base. Top with raspberries. Refrigerate for 3 hours until set. Cut into pieces. Dust with cocoa if desired and serve.