Preheat oven to 180°C/350°F/Gas 4 and line 2 baking trays with non-stick paper.
Put the butter and sugar into a bowl and beat together until light and fluffy. Beat in the egg yolk. Sieve in the flour and spices, add the orange zest and add in enough milk to make a fairly soft dough.
Roll out the dough approximately 5mm thick on a floured surface and cut out tree shapes with the cutter.
Bake for 15 minutes or until the biscuits are a pale-golden brown. Transfer to a cooling rack to cool completely.
Roll out the white icing approximately 3mm thick on a surface dusted with icing sugar. Cut out the same number of tree shapes as you have of biscuits. Paint the back of each icing tree with a little cooled boiled water or edible glue and affix to the tree biscuits. Leave the icing to dry for a couple of hours.
Decorate your biscuits with a variety of different gel food colours. Squeeze a little of each gel colour on a large plate and use a clear alcohol such as vodka to dilute the colour to make a washy paint like a watercolour.
Use a wide, flat brush to add brushstrokes of colour to your biscuits or experiment with different sized brushes.
Mix gold lustre powder with a little vodka to make a paint and use a small paintbrush to flick the colour onto the icing for golden highlights.