Toss cucumbers with ½ teaspoon kosher salt in a colander. Set aside to drain.
Chop peanuts, cilantro and red-pepper flakes together until very fine. Set aside.
In a large bowl, whisk together peanut butter, soy sauce, rice vinegar, sugar, garlic and ½ teaspoon kosher salt. Add up to 1 tablespoon water, if needed, to thin mixture. Transfer salted cucumbers to the bowl with dressing and toss to coat.
To serve, place half of the cucumber salad on the plate, sprinkle with half of the peanut mixture. Repeat with the remaining cucumber salad and peanut mixture. Drizzle with chile oil, if using, and serve immediately.
