Preheat oven to 180 degrees Celsius Grease a 3c deep 16cm x 26cm slice pan. Line base and sides with baking papaer, extending paper 2cm from edge on all sides.
Using an elextric mixer, beat 185gm butter and sugar until light and fluffy. Sift flours over butter mixture. Stir until dough comes together. Press mixture into prepared pan. Bake 20 minutes until golden.
Spread warm base with jam. Cover with marshmellows, cut side down. Bake for 2 minutes. Remove from oven. Press down on the marshmellows to level the surface. Cool in tin.
Combine chocolate and remaining butter in small saucepan over a low heat. Cook, stirring, for 2 - 3 minutes or until melted. Pour over marshmellows. Tap pan to level surface. Refrigerate for 2 hours or until set. Stand at room temperature for 5 minutes before cutting into pieces with a warmed/hot knife. This makes the cutting of the choclate icing easier.