Add mayonnaise, buttermilk, vinegar, sugar, and salt to a large bowl and whisk together with a fork until well-combined and smooth. Stir until sugar is completely dissolved. Pour the dressing over cabbage and toss together to mix thoroughly.
Serve immediately or cover and chill in refrigerator before serving.
Make-Ahead: Mix together coleslaw dressing and store in an airtight container in the refrigerator for up to 3 days. When ready to serve, pour the dressing over the cabbage mixture in a large bowl and stir well to combine. Serve. Vegan: For a vegan version of this coleslaw recipe, use vegan mayonnaise and a plant-based milk.