Slice cucumbers thinly using a sharp knife or mandoline (about 1/8-inch thick).
Place sliced cucumbers in a colander and sprinkle with kosher salt. Toss gently to coat.
Let cucumbers sit for 10 minutes to draw out excess moisture, then pat dry with paper towels.
While cucumbers rest, toast sesame seeds in a dry skillet over medium heat for 2-3 minutes until fragrant, stirring constantly.
In a small bowl, whisk together rice vinegar, sugar, and sesame oil until sugar dissolves completely.
Combine dried cucumbers, dressing, and toasted sesame seeds in a serving bowl.
Toss everything together until evenly coated. Taste and adjust seasoning if needed.
Serve immediately for the crispiest texture, garnished with extra sesame seeds if desired.