For the royal icing:
1) Beat all ingredients together in the bowl of a mixer until stiff peaks form. Food colouring can be added, if desired. Work with the icing in small amounts, keeping the remaining icing in an airtight container, as it will dry quickly. The icing can be re-beaten if necessary.
For the dough:
1) Whisk all the dry ingredients together thoroughly and set aside. Beat butter and brown sugar together in the bowl of a mixer on medium speed until fluffy. Add the eggs, treacle, and water and beat until well combined.
2) Beat half of the flour mixture into the mixer until blended and smooth. Add the remaining flour mixture and then knead the dough until firm and manageable, but not dry. Add more flour if the dough is wet.
3) Place the dough in a sealable container or zip-top bag in a cool place for up to 2 hours. If you make the dough ahead of time and refrigerate it, bring to room temperature before working the dough.
4) When ready to roll out the dough, preheat the oven to 180C/Gas 4. Spray your baking trays with nonstick spray.
5) Roll out dough and cut into desired shapes, working in small batches and keeping the remaining dough in an airtight container so that it does not dry out. Bake for 11 to 15 minutes, depending on the size of the pieces. The gingerbread will be light brown when it is done. Let cool before icing. Store in an airtight container.