In a large bowl, combine the melted butter, sourdough discard, molasses, sugar, and spices, mixing until fully incorporated.
Add the flour and baking soda, and mix until combined. The cookie dough will be thick and slightly sticky, but don’t worry—it will firm up once chilled.
Divide the dough in half. Place each portion onto a large piece of plastic wrap, wrap tightly, and pat down into a disc shape. This disc shape makes it easier to roll out the dough after chilling. Alternatively, you can cover the dough in the mixing bowl and chill it as is, but shaping it into discs now might make it easier to roll it later.
Chill the wrapped dough discs in the refrigerator for at least 4 hours or up to 2 days. don't skip this step! Chilled dough is easier to work with, holds its shape better, and ensures your cookies bake perfectly. Skipping it will give you flat cookies and a hard to work with dough.
When ready to bake, preheat your oven to 350°F (180°C). Line baking sheets with parchment paper or silicone baking mats and set aside.
Flour a clean work surface, as well as your hands and rolling pin. Remove dough from the refrigerator and roll it out to about ¼-inch thickness. If you divided the dough into disks, start with one disk until finished. Don’t be afraid to use extra flour as needed—the dough can become sticky while working with it. Any flour spots on the dough will bake off.
Use your favorite cookie cutters to cut the dough into shapes. Re-roll any scraps and repeat until all the dough is used. Place the cut-out shapes onto the prepared baking sheets, spacing them about 1 inch apart. These cookies won’t spread much during baking, but they still need a little breathing room.
Bake the cookies for 9–12 minutes, depending on their size. For cookie cutters smaller than 4 inches, bake for about 9 minutes. For larger cookies, bake for about 12 minutes. If your oven has hot spots, rotate the baking sheet halfway through to ensure even baking.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely. Once fully cooled, decorate as desired.