Cook 8 oz. short pasta (such as mini farfalle, medium shells, or elbow macaroni) in a large pot of boiling salted water, stirring occasionally, until slightly more tender than al dente (pasta should be tender but not mushy as it will firm up a little once cooled). Drain in a colander; rinse under cold running water to remove excess starch. Drizzle with a bit of toasted sesame oil (about 1 tsp.; this will prevent sticking) and toss to coat. Set aside.

Toast ⅓ cup toasted sesame seeds in a dry small skillet over medium-low heat, stirring often, until golden brown (like a milky tea) and fragrant, about 4 minutes. Immediately and carefully transfer to a mortar and pestle and coarsely grind until powdery (but not pasty), leaving some whole. (If you don’t have a mortar and pestle, place in a resealable plastic bag and lightly crush with a rolling pin or pan.) Transfer to a large bowl and add 1 cup mayonnaise, ½ cup buttermilk, ¼ cup apple cider vinegar, 2 Tbsp. soy sauce, 1 Tbsp. sugar, 1 Tbsp. toasted sesame oil, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt; whisk to combine. Season sauce with freshly ground pepper and more salt if needed; set aside.

Place one 16-oz. bag prepared coleslaw on a large cutting board; run a knife through 3 or 4 times to make the largest pieces smaller. Transfer to bowl with dressing and add reserved pasta; mix carefully and thoroughly to combine. Taste and season with more salt and pepper if needed. Cover and chill at least 1 hour and up to 12 hours.

Just before serving, finely grate half of zest from 1 lemon directly over slaw. Cut lemon in half and squeeze juice of 1 half over. Season with more pepper and squeeze in juice from remaining lemon half if desired. Gently mix slaw again and transfer to a large shallow bowl or platter. Finely grate remaining zest from lemon over and season with more pepper.