1.Preheat oven to moderate, 180°C. Lightly grease and line 2 oven trays with baking paper.
2.In a large bowl, using an electric mixer, beat butter and sugar together until light and fluffy. Beat in egg.
3.Sift flours together and fold into creamed mixture. Turn onto a lightly floured surface. Knead lightly until smooth. Wrap in plastic wrap, chill 15 minutes.
4.Roll out dough between 2 sheets of baking paper until 4mm thick. Using a fluted pastry wheel. cut dough into 24 rectangles 4 x 5cm. Arrange on trays 3cm apart. Bake 10-12 minutes, until lightly golden. Cool on trays for 10 minutes, then transfer to a wire rack to cool completely.
5.Meanwhile, to make topping; in a small saucepan combine marshmallows and butter. Stir over a low heat until melted and smooth. Mix in icing sugar. Spread coconut on flat plate. Working quickly, spread a little icing along both sides of biscuit, leaving centre strip bare. Dip in coconut, shaking off excess. Spread warmed jam down centre strip. Repeat with remaining biscuits. Allow to set.
Warm jam on high in microwave in 10 second bursts, stirring until easy to spread.