Place poblano peppers on the top rack of the oven and broil on high, turning periodically, until skin is charred, about 10 minutes per side. Transfer to a bowl and cover with plastic wrap to steam. When cool enough to handle, remove seeds and peels. Chop coarsely and set aside.
Meanwhile, bring a large pot of salted water to a boil and add potatoes. Return to a simmer and cook until tender, about 15-20 minutes.
Drain cooked potatoes and return them to the pot over low heat. Add butter and milk. Using a potato masher or wooden spoon, mash until fairly smooth. Stir in chopped roasted poblano, sour cream, shredded cheese, salt, cumin, and pepper.
Filling can be made up to two days in advance.