Position rack in upper third of oven; preheat broiler to high. Line a large rimmed baking sheet with foil. Arrange mini peppers on the baking sheet and broil, flipping once, until the skins are blackened and blistered all over, about 8 minutes. Transfer the peppers to a bowl, cover with plastic wrap and let stand for 10 minutes. Remove and discard stems, skins and seeds.