Melt 100g of unsalted butter, 50g of golden syrup, 50g of black treacle, and 100g of light brown soft sugar over low heat in a medium saucepan. Stir frequently until the sugar dissolves (about 5 minutes). Remove from heat and cool for 10 minutes.

In a large mixing bowl, whisk together 225g of plain flour, 0.5 tsp of bicarbonate of soda, 1 tbsp of ground ginger, 1 tsp of ground cinnamon, and 0.5 tsp of mixed spice. Add a pinch of salt and mix well.

Pour the cooled wet mixture into the bowl with dry ingredients and knead for 2-3 minutes until a cohesive dough forms. If too dry, add a teaspoon of water at a time until smooth but not sticky. Wrap in plastic wrap and chill for 30 minutes.

Preheat your oven to 170°C (340°F). Roll out the chilled dough to about 5mm thick on a floured surface. Cut shapes using Halloween-themed cookie cutters. Place on a parchment-lined baking sheet and chill for another 10 minutes.

Bake the cookies in the preheated oven for 10-15 minutes until edges turn golden brown. Let cookies sit for 5 minutes on the tray before transferring to a wire rack to cool.

In a mixing bowl, combine 100g of icing sugar with a small amount of water to create a thick icing. Split into two portions for outlining and filling. Adjust thickness as needed.

Outline the edges of the cookie shapes with icing, then fill in the centers. Use black writing icing to add details like eyes on the ghosts and bandages on the mummies. Allow icing to set completely before displaying.