Peel the cucumbers, halve lengthwise, scoop out the seeds, and cut each half crosswise into thin slices. Mix the cucumber with the salt and 1 teaspoon of the sugar in a colander and set aside for 10 minutes. Meanwhile, whisk together the remaining sugar, the sesame oil, soy sauce, vinegar, scallions, and chili oil if you’re using it in a large bowl.
Rinse, drain, and pat the cucumbers dry, then toss with the dressing. Cover and let sit in the refrigerator for at least 3 hours or up to a day, stir- ring whenever you think of it. Serve chilled.