For the cookies: Place the flour, bicarbonate of soda and spices into a large bowl. Then rub in the cold cubed butter, until it resembles breadcrumbs. Then using a wooden spoon or silicone spatula, stir in the light brown sugar.
In a separate bowl, lightly beat the egg, golden syrup and treacle together until combined. Then pour into the dry mixture, and stir until combined. Press clingfilm directly onto the surface of the dough and chill in the fridge for 20 minutes.
Preheat the oven to 180C/160C Fan/Gas Mark 4 and line two/three large baking trays with baking paper. Pour the Demerara sugar onto a small plate. Then scoop small balls of the dough out (approx. 40g each), roll into balls between your hands and then roll in the sugar, completely coating the outside. Then place on the baking trays, leaving a little gap between each. You should have 22 cookie balls.
Bake for 12-15 minutes or until they feel firm. Leave them on the baking trays for 10 minutes, and then remove and place them onto cooling racks to cool completely.
For the frosting: Whisk the cream cheese and double cream together until smooth and slightly thickened, then whisk in the icing sugar until the mixture is smooth and creamy. Place in a piping bag.
Pair up the cookies into even sizes, so you have 11 pairs. Pipe the frosting onto the bottom side of half the cookies, and sandwich the remaining cookies on top of each, pressing down slightly to set.