Cook the noodles in salted boiling water until they’re al dente.
While those are in the works, combine the peanut butter and 1 cup kimchi in a food processor, and blend until combined. Add 6 tablespoons of liquid—either a combo of kimchi brine and water (I like half and half), or all kimchi brine, or all water, depending on how much spice and acidity you’re looking for. Blend again, scraping down as needed, until smooth-ish. Taste and add salt if needed, then transfer to a big heatproof bowl.
When the noodles are done, drain them into a colander in the sink and rinse very well with cold water. Shake a couple times, then add the cold noodles to the sauce. Toss until coated, then add more kimchi brine or water, if needed, to get the sauce to the consistency you want.
These are best as soon as they’re tossed, but they also hold well at room temperature for a couple hours. Or refrigerate for later. (Just keep in mind that the sauce will thicken further in the fridge—you can thin with kimchi brine or water as needed.) Whenever you serve, pile the remaining 1 cup kimchi on top of the noodles in the serving bowl or divided on top of individual portions.