Lightly grease two baking trays.
Sift the flour and cocoa into a bowl.
Rub in the butter or margarine until the mixture resembles fine breadcrumbs.
Stir in the sugar, then bind the mixture with the golden syrup and enough beaten egg to give a firm dough.
Knead lightly, then roll out the dough on a floured board to a rectangle measuring about 30 x 40 cm/12 x 16 in.
Trim the edges and cut the rectangle into 32 biscuits measuring 2.5x7.5 cm/1 x 3 in.
Using a palette knife, carefully transfer the biscuits to the prepared baking trays.
Chill for 1 hour, or longer if you have time.
Bake in a moderate oven (180C, 350F, gas 4) for 10-15 minutes, or until darkened in colour and cooked.
Sprinkle the biscuits with sugar immediately they are removed from the oven.
Leave to cool on the trays for a minute before transferring them to a wire rack to cool completely.
While the biscuits are cooling, make the filling according to the recipe instructions.
Sandwich the biscuits together in pairs with the chocolate butter icing.