In the bowl of a stand mixer or large bowl, if making by hand, add the sifted flour, yeast, sugar, oil, sesame seeds, and eggs. Stir or use the dough hook to combine ingredients.
With the mixer on low, slowing pour in the water adding enough for the dough to come together. You may need more or less than 2 cups depending on the dough feel. It should be tacky but not too sticky. If it's too sticky and wet, add a tbsp more flour. Continue to adjust until the dough is the tacky consistency.
Knead in the salt using the dough hook or by hand and continue to knead 6-8 minutes with the dough hook or 10 minutes by hand.
Remove dough hook and punch down dough into the large bowl. Cover with plastic wrap sprayed with cooking spray leaving the plastic loose at the top as the dough will likely rise above the bowl. Ensure the plastic is secure around the bowl to prevent drafts. Place the bowl in a warm area for 1-1½ hours or until doubled in size.
Deflate the dough and scoop it out onto a lightly floured work surface. Knead a few times until it all comes together and is deflated. Knead it into a nice round ball. Cut it in half and set one half aside. From these 2 halves, you can make 2 challahs which will be very large and include 6 braids each. Or you can make 4 challahs which will be much smaller and made with 3 braids each.
To form the challahs, divide the large dough in half and set one aside. Take one of the two halves and divide it in half again. Divide these new two halves into thirds. This will give you 6 balls of dough for 1 challah or 3 balls of dough each for 2 challahs from this one half.
Roll out each of the 6 balls of dough into ropes about 12" long each. You can use all 6 ropes to braid into 1 challah or divide them and use 3 ropes to make 2 separate challahs.
If making a 6-braid challah, line all 6 dough ropes side-by-side touching. Pinch 1 end together to seal that end so the ropes are all connected. Take the strand on the far right and lay it past the 2 strands next to it towards the center. Take the 2nd strand from the left and move it to the far right. Take the outside left strand and move it towards the center past the 2 strands next to it. Continue until the challah is fully braided. Remember to manipulate each strand before returning to the 1st strand and so forth. Once the braid is complete, pinch and seal the braid tightly at that end. Adjust the braiding technique to fit your skill level and desired aesthetics of the bread. The key is to have uniform thickness from one end to the other with a tight intertwining of ropes from end to end. Be sure both ends are sealed tightly to prevent the loaf from separating as it rises and bakes.
If making a 3-braid challah, line 3 dough ropes side-by-side touching. Pinch 1 end together to seal that end so the ropes are all connected. Then just simply intertwine the 3 ropes to form a braid all the way down to the other end. Once the braid is complete, pinch and seal the braid tightly at that end. Adjust the braiding technique to fit your skill level and desired aesthetics of the bread. The key is to have uniform thickness from one end to the other with a tight intertwining of ropes from end to end. Be sure both ends are sealed tightly to prevent the loaf from separating as it rises and bakes. Repeat these steps for the other 3 ropes.
Repeat the same steps with the other half of dough that you set aside before making the 3 or 6-braid challahs.
Place the 2 or 4 challahs on 1 or 2 baking sheets lined with parchment paper spacing them 2"-3" apart to allow for rising and to expand while baking.
Cover the challahs with plastic wrap sprayed with cooking spray and set them aside in a warm area to rise and puff up for 30 minutes.
At the 15 minute mark into the rise, preheat the oven to 350˚F / 177˚C.
While the oven is preheating, make the topping. In a small bowl, whisk the egg yolks and water and set aside until the rise time is complete.
Once the challahs have puffed up, carefully remove the plastic wrap and lightly brush on the egg yolk and water mixture covering tops and sides of all challah loaves. Sprinkle on sesame seeds, as much as you'd like.
Place the challahs in the oven to bake. If baking 3-4 challahs, check for doneness at 20 minutes. If baking 2 challahs check for doneness at 30 minutes. Regardless of the number of loaves, the challahs should be dark brown and crusty on top. If you have a thermometer, check that the internal temperature of each is at least 190˚F / 88˚C to ensure all bread loaves are done. Doneness guide: 3-4 challahs will take 20-30 minutes to bake while 2 challahs will take 30-40 (maybe a little longer) minutes to bake. Check internal temperature to be sure.
When bread is done, you can let it cool or go ahead and dig in. Slice and store in freezer for up to a couple of months and take out slices as you want them.