1. In mixer, add warm water, 1 Tb. Xylitol, yeast, mix for a minute and let sit until frothy.
2. Add rest of Xylitol and honey and mix to incorporate, using a rubber spatula as needed.
3. Add seltzer water, half of flour, salt, oil, and mix for a few minutes, scraping sides of bowl occasionally.
4. Add rest of flour, mixing until the dough is formed into a ball.
5. Remove the lid of mixer, push down dough until even, then cover mixer with Saran wrap. Let rest for 30 minutes.
6. Mix dough for a minute to get dough loosened from paddle. Remove dough and hand knead for a few minutes on a lightly floured board, to form a ball shape.
7. Lightly coat a large bowl with shortening.
8. Place dough in bowl and turn it over, which will cover the top part of dough with shortening.
9. Cover with Saran wrap and let rise in warm area of room until the dough rises to double in size.
10. Take Challah, then shape dough portions into bread pans. The dough should be at 2/3s height of bread pan. Let rise until double.
11. Preheat oven to 400 degrees.
12. Brush each challah with egg wash and top with seeds if using.
13. Bake for 10 minutes at 400 then reduce temperature to 350 and bake until medium golden brown, approximately an additional 15 to 25 minutes depending on the size of the bread pans.
14. Remove from oven and cool 5 minutes, then remove from bread pans. Let cool on racks.
15. Cool thoroughly before wrapping and freezing.
16. Alternatively, you can freeze any unbaked dough in portions–in double wrapped plastic bags.