Char the peppers over a gas burner or a grill.  When the skin is blackened, place the peppers in a bowl and cover tightly with plastic wrap. Let sit until cool.  The skin should slide off or rub off easily.  Remove skin and seeds and chop the peppers.

Grill the corn until charred, then cut the kernels off the cobs.

Quarter the potatoes and place in a large pot. Cover with water, then place over high heat and bring to a boil. Cook until tender, about 12 minutes.  Drain and let cool.

Combine the avocado, buttermilk, cilantro, and garlic in a food processor or blender.  Process until smooth.  Add the lime juice, then season to taste with salt.

In a large bowl, combine the chopped peppers, corn, potatoes and avocado cilantro dressing.  Stir to combine.  Season with more salt, if needed. 

Refrigerate at least 2 hours before serving.