1Preheat oven at 180 C. Lightly grease and line 2 oven trays with baking paper.
2In a large bowl, using an electric mixer, beat butter and sugar together until light and fluffy. Beat in egg.
3Sift flours together and fold into creamed mixture. Turn onto a lightly floured surface. Knead lightly until smooth. Chill 15 mins in plastic wrap.
4Roll out dough between 2 sheets of baking paper until 4 mm thick. Using a fluted pastry wheel, cut dough into 24 rectangles 4 x 5 cm in size. Arrange on trays 3 cm apart. Bake 10 to 12 mins until lightly golden. Cool on trays for 10 mins, then transfer to a wire rack to cool completely.
5To prepare topping, combine marshmallows and butter in a saucepan. Stir over a low fire until melted and smooth. Mix in icing sugar. Spread coconut on a flat plate. Working quickly, spread a little icing along both sides of the biscuits, leaving centre strips bare. Dip in coconut, shaking off excess. Spread warmed jam down centre strip. Repeat with remaining biscuits. Allow to set and store in airtight containers.