Pre-heat oven to 180°C (160°C fan-forced). Line an 18 x 27cm slice tray with baking paper, extending beyond the tray
Combine flour, sugar and baking powder in a mixing bowl. Add butter and mix until well combined. Press into the prepared tray and bake for 20 minutes. Allow to cool.
Place chocolate and cream in a heatproof bowl over a saucepan of gently simmering water, stirring regularly until chocolate has melted. Pour chocolate mixture over the biscuit base in the slice tray and refrigerate until set, approximately 1 hour.
For the marshmallow, place the ¼ cup of water in a small bowl and sprinkle the gelatine over it, set aside. Combine the sugar with the 1 ½ tablespoons of water in a small saucepan. Bring to the boil, ensuring all sugar has dissolved. Continue on a simmer for approx. 8-10 minutes, or until it has reached 115°C. Remove from heat and stir in the gelatine mixture.
Meanwhile, beat egg white in the bowl of an electric mixer, until firm peaks form. Gradually add the sugar mixture in a steady stream until all incorporated. Continue mixing for approximately 10 minutes until mixture is thick and glossy and the bowl feels cool.
Spread the marshmallow mixture over the chocolate layer and then using a blow torch, torch the marshmallow until golden. If you don't have a blow torch, place under a pre-heated grill until golden.