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Rub the malt between palms and squeeze out milky liquid. Strain the malt liquid through a fine sieve into another bowl. If you don’t have a fine sieve, line a strainer with a cheesecloth. Then, squeeze the malt husks with your hands through the sieve to drain out the remaining liquid. Transfer the liquid to a large heavy bottom pot.
Place the malt husks in the first bowl and add another 10 cups of water. No need for an additional soaking time. Repeat the squeezing and straining process. Transfer the liquid to the pot with the first batch of milky liquid. This will yield about 17 to 18 cups of malt water.
Turn the heat on to medium and boil the liquid down to a half, stirring once in a while. Measure the depth of the liquid with a wooden spoon or spatula, so you can easily tell when the liquid is reduced to a half. This will take about 2 hours and the resulting liquid will be around 8 to 9 cups. You’ll need about 8 cups for this recipe, but a little bit of extra is nice to have just in case. If you're using other optional liquid, you can reduce it further. Stir once in a while. Let it cool down a bit.
Stir-in the gochugaru and mix very well until there are no big lumps. Continue to stir. You can use a large wooden spoon or spatula this time. If the gochujang is too thick, use more malt liquid or optional rice syrup (jocheong), plum extract, and/or soju to adjust the thickness and sweetness. Let it sit for a few hours until the dry ingredients fully absorb the liquid, and salt is completely melted.