Crispy Chicken and Potatoes with Mustard

Ingredients

• 1 whole chicken, about 4 pounds, quartered
• 2 tablespoons unsalted butter, room temperature
• 3 tablespoons grainy mustard
• 1 tablespoon chopped fresh rosemary
• 1 tablespoon chopped capers
• 3 tablespoons olive oil
• 1 1/2 pounds small roasting potatoes or larger potatoes cut into 1 ½-inch chunks
• Kosher salt and freshly ground pepper