Place the chicken breasts in a medium baking dish.
In a small bowl, add the mustard, honey, oil, garlic powder, rosemary and salt and stir together with a fork. Pour the honey mustard mixture over the top of the chicken breasts, then use a spoon to spread it around. Make sure all the chicken is covered.
Wearing oven mitts, place the baking dish in the hot oven and bake the chicken for 25-30 minutes, or until it is cooked through. To check, wearing oven mitts, remove the dish from the oven and put it on the potholder. With a paring knife, cut through the thickest part of the breast. If it is white all the way through, it's done. If any pink remains, return the baking dish to the oven for 5 minutes more and check again.
Sprinkle the chicken with a little black pepper and serve, drizzled with some of the pan liquid.