Thinly slice cabbage and then roughly chop to cut into shorter pieces.
Place shredded cabbage in a strainer in the sink and sprinkle with 2 teaspoons of kosher salt. Toss with the salt and let sit for 2 hours. (See Note 1)
When the time is up press down on the cabbage to remove any excess moisture and then rinse with cold water. Pat the rinsed cabbage dry with a paper towel.
Place shredded cabbage and carrots in a large bowl.
In a small bowl make the dressing by combining the mayonnaise, buttermilk, sugar, lemon juice, onion powder, white vinegar, salt, and pepper. Stir until well combined.
Pour the dressing over the cabbage and carrot mix and toss together.
Cover bowl with plastic wrap and place in the refrigerator to chill for 1 hour.
After coleslaw is chilled stir well and serve with a slotted spoon to strain out any excess liquid.
Notes
Steps 2 & 3 are not required if you are short on time. If you are not able to let your cabbage sit for 2 hours simply move to step 4 and toss the cabbage and carrots with 1 teaspoon of salt.