Mix together the water and yeast. Set aside while you prep the other ingredients.
In a large bowl, mix together the flour, white granulated sugar, and kosher salt.
Add the yeast water to the dry ingredients and mix with a wooden spoon.
Add the vegetable oil to the dough and mix properly. It will be sticky but keep mixing!
Roll the dough into a ball and place into a greased bowl and cover with cling wrap.
Place the dough aside in a warm/room temperature room for an hour as it rises.
Once the dough has risen for an hour, punch the gases out of it and roll onto a floured work surface (use the ⅛ cup of flour for this).
Knead the flour into the dough until fully incorporated and the dough feels smooth. It's okay for it to be sticky.
Cut the dough into four equal sized lumps and cover with the cling wrap you used earlier. Set aside while you mix the filling and chop the nuts.
Finely chop the walnuts and then measure out a tablespoon of them. Mix with the cinnamon and dark brown sugar.
Flatten each of the dough balls slightly in the palm of your hands and then evenly add the stuffing between the four dough pieces.
Refer to the photos above on how to seal the dough balls. Set them aside.
Depending on the size of your pans, use enough vegetable oil to thinly cover the bottom of whatever pan you're using. It can be cast iron, enameled cast iron, nonstick, etc. Let it heat up until shimmering.
Place one or two of your dough balls in the pan (the pan should sizzle). Make sure there's enough room in the pan for the balls to expand as you flatten them.
Let the balls cook for about a minute to a minute and a half or until the dough is golden, crispy brown.
Flip the dough and take a flat spatula, a burger flipper, or something like that as well as a heavy handled spoon. Place the flat spatula on the dough and use the wooden handle to push on the spatula to flatten the dough. It should be flat like a disc.
Cook the hotteok for another minute to minute and half, or until its golden crispy brown.
Flip the hotteok one more time and let the rest of it cook before removing from the pan and placing on some paper towels.
Enjoy warm! If you want to make ahead and reheat, you can heat them up on a pan or in a toaster oven!