In a stand mixer or a large bowl, mix the sourdough starter with 1 cup bread flour and water. Cover the bowl tightly and let it stand overnight. If it is very warm where you are, leave it in the refrigerator overnight and let it stand on the countertop for another hour before proceeding.
To the bowl, add the remaining 1 cup of bread flour and 1 cup of all purpose flour. Add the aquafaba, maple syrup and olive oil and knead until the dough comes together in a ball. Continue to knead at medium-low speed for 10 minutes in the stand mixer, or knead by hand. If the dough feels tacky, add more flour, a tablespoon at a time. You want a firm but smooth and elastic dough. A soft dough will not hold its shape after rising, so this is important.
Shape the dough into a smooth ball and place in an oiled bowl, turning over once to coat the top with oil. Cover with cling wrap and place in a warm spot, like the oven with the pilot light turned on, for two hours or until the dough has doubled.
Remove the dough to the countertop and knead it briefly. Divide it into eight balls and roll each out into a long strand. You can braid these to either form long loaves or round loaves like I did. I used the four-braid method to shape my two challah loaves (see recipe notes for the video I watched to shape these).
Line a baking sheet with parchment paper. Place the shaped challah loaves on the sheet, at least five inches apart. Cover loosely with cling wrap and let them rise in a warm spot for an hour.
About half an hour before baking, preheat your oven to 350 degrees.
Place the challah loaves in the hot oven and bake 35 to 40 minutes. Remove from the oven and continue cooling on a rack.
To get a really great color on your challah loaves, mix 1 tsp of olive oil with 1 tbsp of maple syrup and 1 tbsp of aquafaba to make a wash. Brush this on the top of the loaves, once as soon as you've shaped them and placed them on the baking sheet, and once again just before baking.