Heat a grill or broiler to medium-low. Rub the peppers lightly with the canola oil and put on a sheet pan. Roast the peppers until blackened, then transfer them to a paper bag or thick plastic bag to steam. When cool enough to handle, peel and remove the seeds, then dice. Set aside.
Add the potatoes to a large pot and cover with salted water. Cook the potatoes over medium-low heat until fork tender, about 8 to 10 minutes. When tender, drain and return the potatoes to the pot. Add the sour cream, milk, butter, pepper jack, salt, pepper, and the diced peppers. Mash by hand with a potato masher and adjust seasoning, if needed. Hold warm until ready to serve. Transfer to a serving bowl and garnish with cilantro and cotija, if desired.