How to Make It
Preheat oven to 400°F. Toss together walnuts, butter, cayenne, and ½ teaspoon of the salt in a medium bowl. Spread in a single layer on a rimmed baking sheet and toast until golden and fragrant, about 5 minutes.
Whisk together buttermilk, mayonnaise, basil, scallion, black pepper, and remaining ½ teaspoon salt. Cover and chill until ready to use. (Dressing can be stored in the refrigerator for up to 3 days.)
Toss together lettuce, radishes, and 3 tablespoons of the dressing in a large bowl. Top with walnuts. Serve with remaining dressing on the side.