Gently melt the butter, muscovado sugar and syrup in a large pan until the sugar dissolves. Mix the flour, spices and ½ tsp salt. Cool the butter mixture a little, then stir in the bicarb. Immediately add half the spiced flour and beat well. Add the egg and the rest of the spiced flour, then beat well again until the mixture comes together as a soft dough. Tip onto a sheet of foil, flatten to a large disc, then cool and chill until firm. Will freeze for up to 6 weeks.
Heat oven to 190C/170C fan/gas 5. Halve the dough and roll out on a floured surface. Stamp out trees using a cutter about 10cm long and arrange, well spaced apart, on baking sheets. Bake for 12-15 mins until golden. Leave to harden, then lift onto a rack. Repeat with the remaining dough. Use the trimmings to stamp out 3-4cm stars, then bake for 9-10 mins.
To decorate, mix about 3 tbsp water into the icing sugar to make a thick icing. Use to sandwich two trees together with a lolly stick between them. Use the rest of the icing to ice the trees and stars, adding sweets or sprinkling with the sugar. For a neater result, use the writing icing to make an outline of the tree before filling in with the icing. Leave to set. Will keep in a tin for 3-4 days.