1. In a saucepan set over low heat, or in the microwave, melt butter, then stir in the brown sugar, molasses, salt, and spices.
2. Transfer the mixture to a medium-sized mixing bowl, let it cool to lukewarm, and beat in the egg.
3. Whisk the baking powder and soda into the flour, and then stir these dry ingredients into the molasses mixture.
4. Divide the dough in half, and wrap well. Refrigerate for 1 hour or longer.
5. Preheat your oven to 350°F. Get out several baking sheets; there's no need to grease them, though lining with parchment saves effort on cleanup.
6. Once the dough has chilled, take one piece of dough out of the refrigerator, and flour a clean work surface, and the dough. Roll it out as thin or thick as you like; for slightly less crisp cookies, roll it out more thickly.
7. Use flour under and on top of the dough to keep it from sticking to the table or rolling pin. Alternatively, place the dough on parchment, and put a sheet of plastic wrap over it as you roll, pulling the plastic to eliminate wrinkles as necessary when rolling; this will keep dough from sticking without the need for additional flour. For soft dough, or dough to be rolled extra-thin, you may choose to roll right onto the ungreased back of a baking sheet.
8. Cut out shapes with a cookie cutter, cutting them as close to one another as possible to minimize waste.
9. Transfer the cookies to ungreased cookie sheets (or, if you've rolled right onto the parchment, remove the dough scraps between the cookies). Bake the cookies just until they're slightly brown around the edges 8 to 12 minutes, or until they feel firm. Let the cookies cool on the baking sheets for several minutes, or until they're set. Transfer them to a rack to cool completely. Repeat with the remaining dough.