Preheat the oven to 180°C. Grease a 27x18cm slice tin and line with baking paper.
Sift flour, baking powder and cocoa into a bowl. Stir in caster sugar and coconut. Add butter and egg, mixing well. Spread over the base of the prepared tin.
Bake for 20-25 minutes. Cool for 15 minutes and then spread with topping.
While the cake is cooling make the topping by placing chocolate and Kremelta in a heatproof bowl. Sit over a saucepan of simmering water. Stir continuously until smooth. Set aside for 5 minutes to cool slightly.
Add marshmallows, coconut and nuts to the melted chocolate. Mix well. Spread over the warm base. Allow topping to set before cutting into pieces.