Beat together the butter and sugar until pale and creamy. Add the egg and treacle. Sift the flour and ginger into the bowl and mix all the ingredients to form a firm dough. Knead lightly and chill for 30 minutes.
Grease lightly 2 baking sheets. Roll out the dough to a thickness of 10 cm (½ inch) and cut out heart shapes, using a biscuit cutter. Reroll the trimmings to make more biscuits. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 10 minutes until slightly risen. Transfer to a wire rack and leave to cool.
Melt the plain dark and milk chocolate in separate bowls over pans of simmering water (see
Put the reserved milk chocolate in a piping bag and with a fine nozzle pipe wavy lines over the biscuits covered with plain dark chocolate. Use the reserved plain dark chocolate to decorate the biscuits covered with milk chocolate. Leave to set.