Line a large baking sheet with parchment paper.

In a medium glass bowl or measuring cup, combine the yeast, 1½ teaspoons of
the sugar, and the lukewarm water. Leave for 10 minutes, or until
foamy.

Meanwhile, in the bowl of a stand mixer fitted with a dough hook, mix the remaining 4½ teaspoons sugar, 1 cup of cool water, the kosher salt, and the oil on medium low speed for 1 minute.

Add the yeast mixture to the mixing bowl, beating well.

With the mixer on low speed, add the flour.

Raise the speed to medium and knead for 4-5 minutes, until a smooth, satiny dough forms.

Place in a lightly oiled bowl and cover with a clean dish towel and allow the dough to rise in a warm place for 1½ hours, or until doubled in size.

Transfer the dough to a lightly floured surface. If the dough is sticky, knead
in more flour, a little bit at a time, until the dough is easy to roll.

Divide the dough into six equal balls and roll each into a long
strand. Braid three strands of dough into a challah, for a total of two
challahs. Place on the prepared baking sheet.

Preheat the oven to 350 degrees.

Bring 8 cups of water and the baking soda to a boil in a pot with a wide
opening. Gently and carefully lower a challah into the baking soda
solution. Using two wooden spoons or spatulas, carefully turn the
challah so both sides get equally covered in water. Remove after 30
seconds and place back on the parchment lined pan. Repeat with the
other challah.

Brush the top of each challah with some water from the pot and then top with salt or sesame seeds.

Bake for 30 minutes, or until starting to brown. Remove from oven and cool completely.