On a large cutting board, slice cucumber in half lengthwise and place them cut-side down. Using a large chef's knife or cleaver, press firmly on the cucumber halves until cracked and broken in several places. (If you want to get some aggression out, you can also whack the cucumbers with a cleaver or even a heavy skillet.) Cut smashed cucumbers crosswise into ¾-inch slices and transfer to colander set inside a bowl. Sprinkle with salt and toss to coat; refrigerate 15 minutes.
While cucumbers drain, make dressing. In a small bowl, whisk together vinegar, chili oil, sesame oil, and sugar until sugar dissolves and dressing is emulsified.
After 15 minutes, remove cucumbers from refrigerator and discard accumulated liquid. Transfer cucumbers to same bowl. Drizzle dressing over cucumbers and toss to coat. Divide among 4 plates or transfer to platter. Garnish with sesame seeds and cilantro; serve.
Though rice vinegar works in a pinch, Chinkiang vinegar is worth seeking out!
To toast raw sesame seeds, cook and stir in a dry skillet over medium heat for 3 to 5 minutes until lightly brown and fragrant. Transfer to a small bowl immediately to keep from over-browning.