In a small, combine flour, salt, baking soda, and spices.
In a large mixing bowl with an electric mixer (or the bowl of a stand mixer fitted with a paddle attachment), cream butter, brown sugar, and molasses until it's thick and fluffy.
Beat in the egg, vanilla and the sourdough starter until combined.
Add the dry ingredients to the wet ingredients and use a rubber spatula to combine them into a thick, sticky dough.
Shape the dough into a mound and cut it into two pieces. Wrap the pieces in plastic wrap and chill them in the fridge for at least 3 hours or up to 36 hours.
Rolling
Preheat the oven to 350°F /180°C.
Unwrap one of the pieces of cold dough and place it on a floured workspace. Roll it out until it's ¼ inch thick, dusting the top of the dough to stop it from sticking to the rolling pin.
The initial rolling will be harder, and the dough will crack. It will get easier to roll each time as the butter in the dough will soften.
Use as much flour as you need to roll it out without sticking.
Use cookie cutters to cut out dough shapes and place them on a baking sheet lined with parchment paper, spaced around an inch apart. If you're cutting different-sized cookies, bake the same-sized cookies on the tray together so they bake evenly.
Gather all the dough scraps, re-roll, and continue cutting.
Bake the cookies for around 8-10 minutes, depending on their size., until lightly browned around the edges. For crispier cookies, you can bake them a few minutes longer.
Once baked, remove the cookies from the tray and place them on a wire rack to cool.
Icing
To make the icing, add an egg white to a medium bowl and use a hand mixer to beat it at medium speed until it is frothy.
Add the powdered sugar and beat it on low speed until combined. Turn the mixer onto medium-high speed and beat for 3-4 minutes until thick and glossy. If the icing is too thick, add a few drops of water as needed.
Add the icing to a piping bag and decorate the cookies as desired.
Notes
Both metric measurements and US cup conversions are provided. Use the toggle at the top of the recipe card to switch between the two.