Preheat the oven to 425 degrees. Line a rimmed baking sheet with foil. In a deep skillet, heat 3 tablespoons butter over medium heat. Add the garlic and cook, stirring frequently, until golden, about 3 minutes. Using a slotted spoon, transfer the garlic to a small bowl. Add the onion to the skillet and cook, stirring occasionally, until golden, about 8 minutes.
Add the chipotle chiles in adobo sauce to the skillet. Add the ketchup, vinegar, brown sugar, worcestershire and garlic and cook, stirring occasionally, until thickened, about 5 minutes.
Using tongs, coat the chicken in the sauce and place skin side up on the prepared baking sheet. Bake until the chicken is cooked through, about 30 minutes.
Meanwhile, in a large saucepan, cover the potatoes with salted water and bring to a boil. Lower the heat and simmer until tender, about 12 minutes. Drain, then mash with the remaining 3 tablespoons butter. Serve alongside the chicken.