1) In a medium-sized bowl, beat together the butter, shortening, sugars, baking powder, spices, and salt until light and fluffy.
2) Add the egg and molasses, and beat well.
3) Mix about half of the flour into the butter mixture. When well combined, add the cornstarch and the remaining flour.
4) Divide the dough in half, flattening each half slightly to make a disk. Smooth the edges by rolling the disk along a lightly floured work surface. Wrap in plastic, and refrigerate for 1 hour (or longer), for easiest rolling.
5) Preheat the oven to 350°F.
6) Take one piece of dough out of the refrigerator, and flour a clean work surface, and the dough.
7) Roll it out as thin or thick as you like. For slightly less crisp cookies, roll it out more thickly. We like to roll these cookies 1/8" to 1/4"inch thick. Use flour under and on top of the dough to keep it from sticking to the table or rolling pin.
8) Alternatively, place the dough on parchment, and put a sheet of plastic wrap or another piece of parchment over it as you roll, pulling the plastic or parchment to eliminate wrinkles as necessary when rolling; this will keep dough from sticking without the need for additional flour.
9) Transfer the cookies to ungreased or parchment-lined baking sheets.
10) Bake them just until they're slightly brown around the edges, or until they feel firm, about 10 to 12 minutes.
11) Remove the cookies from the oven, and let them cool on the baking sheet for several minutes, or until they're set. Transfer them to a rack to cool completely. Repeat with the remaining dough.
Yield: 3 to 4 dozen 2 1/2" to 3" cookies.