1) Whisk together the flour, cheese powder, yeast, salt, and baking powder. If you're using baker's ammonia in place of baking powder, set it aside.
2) Add the shortening, working it in to make an unevenly crumbly mixture.
3) If you're using baker's ammonia, dissolve it in a tablespoon of the ice water, and sprinkle it over the dry mixture before adding the remaining ice water. Whether you're using baking powder or baker's ammonia, stir and toss in enough of the ice water to make a cohesive (but not sticky) dough.
4) Divide the dough in half, and shape each half into a small rectangular slab.
5) Cover with plastic wrap, and refrigerate for 30 minutes, or up to a couple of hours; don't chill longer than that. Preheat the oven to 400°F.
6) Very lightly flour a piece of parchment, your rolling pin, and the top of the dough. Working with one piece at a time, roll the dough about 1/16" thick, or slightly thicker. If you don't have parchment, roll on a lightly floured work surface or silicone rolling mat. The dough will have ragged, uneven edges; that's OK. Just try to make it as even as possible.
7) If you've used parchment, gently slide the parchment and crackers onto a baking sheet. Cut the dough into 1 1/4" squares; a rolling pizza wheel works well here. Don't separate the squares. If you haven't used parchment, gently fold the rolled dough in half, pick it up, and place it on a lightly greased cookie sheet, THEN cut it.
8) Prick each square with the tines of a fork.
9) Bake the crackers for about 8 minutes, till the ones on the outside are starting to brown around the edges. Remove them from the oven, and transfer the browned crackers to a cooling rack or piece of parchment; they're done. Quickly and carefully pull the remaining crackers apart to separate them. Return to the oven.
10) Bake for an additional 3 minutes or so, or until the remaining crackers are a very light golden brown. You'll need to watch these closely at the end; don't walk away from the oven. They go from golden to dark brown very quickly.
11) Remove them from the oven, and cool right on the pan. When completely cool, store in an airtight container.