Make the dressing: Whisk the vinegar, mustard, sugar, cayenne, 1/4 teaspoon salt and a few grinds of black pepper in a large bowl. Slowly drizzle in the peanut oil, whisking constantly.
Make the salad: Add the kale and cabbage to the bowl; toss and massage with your hands until the kale softens, about 1 minute. Add the peanuts, cilantro and scallions and toss to combine. Season with salt and pepper.
Photograph by Ryan Dausch