Put the butter, sugar and golden syrup in a wide pan, place over a low heat and stir until the sugar has dissolved. Remove from the heat and stir in the bicarbonate of soda.
Mix together the flour and cocoa powder, and sift half of this mixture into the sugar mixture. Add the beaten egg, stir to mix and sift in the remaining flour mixture. Stir until the mixture forms a soft dough, then spread onto a foil-lined baking sheet.
Flatten the dough into a large circle, cover and chill for 3-4 hours or until firm. Preheat the oven to gas 5, 190ºC, fan 170ºC. Cut the dough in half and roll out each piece on a floured work surface.
Using a 10cm (4in) tree cutter, cut out 36 trees and place on baking sheets lined with nonstick baking paper. Bake in the oven for 10-12 minutes, then cool. To decorate, melt the white and plain chocolate in two separate bowls over pans of gently simmering water. Spread each mixture onto 2 large plates.
Dip one side of half the biscuits in white chocolate, then sprinkle over the dark chocolate sprinkles. Dip the other half of the biscuits in plain chocolate; sprinkle with white chocolate chips. Leave to set.
Freezing and defrosting guidelines
Bake according to the recipe but stop before adding decoration or icing if called for. Allow to cool completely. Wrap well in clingfilm and tin foil (to prevent freezer burn) and freeze for up to 3 months.
Loosen the tin foil and clingfilm but leave to defrost in packaging at room temperature for several hours before serving. If you wish to serve the baked goods warm, place them on a baking sheet for 10 minutes in a moderate oven.
If you wish to add icing or decoration, add them after the baked goods have fully defrosted and are at room temperature.