Pour the dark and light molasses into a large saucepan with the sugar, butter, zest, and spices and melt over low/medium heat, stirring frequently until the sugar has dissolved. If using honey, be aware that it can cause your dough to spread a little when baked.
Increase the heat to bring the mixture to boiling point.
Remove from the heat and beat in the baking soda. The mixture will froth up at this point as the baking soda react. Mix briefly until combined, then let cool for 15 minutes.
Sift the flour and salt, then fold into the mixture in batches, using a wooden spoon or a stand mixer. Beat in the egg using a wooden spoon or stand mixer, until just combined. Do not overwork the mixture, or the cookies will spread during baking.
The dough will be very sticky to begin with, but do not add any flour. Scrape out of the bowl onto a clean surface and knead together until just smooth. Wrap in plastic wrap and chill in the fridge for 1 hour.
Used with permission from Gingerbread Wonderland: 30 Magical Houses, Cookies, & Cakes, by Mima Sinclair.