For the Dressing: In a small bowl, whisk together sour cream, buttermilk, vinegar, mayonnaise, sugar, lime juice and zest, cilantro, garlic, cumin, and black pepper.
For the Slaw: Combine cabbage, carrot, and jalapeño in a large bowl. Sprinkle with sugar and salt and toss to combine. Let stand 5 minutes, then transfer to a large colander and rinse thoroughly under cold running water.
Transfer vegetables to a salad spinner and spin dry. Alternatively, transfer to a large rimmed baking sheet lined with a triple layer of paper towels or a clean kitchen towel and blot mixture dry with more towels. Return to large bowl.
Pour dressing over vegetables and toss to coat. Adjust seasoning to taste with salt, pepper, and/or sugar.
