Make the spicy gingerbread by beating the butter and sugar together until creamy. Stir in the egg and black treacle. Sift in the flour and ground ginger and stir to form a stiff paste. Turn out the dough on to a lightly floured work surface and knead lightly until smooth. Wrap and chill for at least 30 minutes before using.
Roll out the dough thinly on a lightly floured surface and cut out rounds using a 6 cm (2½ inch) round biscuit cutter. Place on greased baking sheets, spaced apart, and re-roll the trimmings to make extras. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 12 minutes or until the dough has risen slightly and is darkening at the edges. Transfer to a wire rack to cool.
Beat together the butter and a little of the sugar. Gradually beat in the remaining sugar and the milk or cream until smooth. Use to sandwich the cookies together in pairs.
Make templates of a simple Christmas tree and star from 7 cm (3 inch) circles of paper. Lay a template over a cookie and dust with plenty of icing sugar. Carefully lift off the template, shake off excess sugar and repeat the decoration on the other cookies.