How to Make It
Preheat oven to 450°F. Toss together jalapeño, tomatillo, garlic, oil, 1/4 teaspoon of the salt, and teaspoon of the pepper on a rimmed baking sheet. Bake in preheated oven until tender, about 12 minutes. Cool completely, about 30 minutes.
Process jalapeño, tomatillo, garlic, and cilantro in a food processor until finely chopped, about 10 seconds. Add mayonnaise, buttermilk, lime juice, cumin, and remaining teaspoon 3/4 salt and 3/4 teaspoon black pepper; process until smooth, about 10 seconds.