Preheat oven to 450˚F. Line 8x8" cake pan with 2 layers of foil paper.
In a small bowl, mix mustard, maple syrup, vinegar, and pepper.
Pour ¼ of the sauce into the pan and add breasts on top. Pour remaining sauce on top, making sure that the chicken is fully covered with sauce.
Bake 25-30 minutes uncovered until chicken reaches 165°F on a digital thermometer. Mine cooked perfectly in 25 minutes for smaller sized breasts.
Remove from oven and allow to rest for 5 minutes. Remove chicken from pan (onto plates).
Add cornstarch (if using) to the juices in the pan to thicken, and mix with a fork or whisk.*
Spoon sauce on top of the chicken and sprinkle with rosemary.
Notes
* I use the full 1 ½ tablespoons of cornstarch because I like a little thicker sauce. If the sauce is not thickening, transfer juices to a pot and heat until bubbling and thickened.
For Dijon chicken thighs, cook the same way, but a bit longer, about 40 minutes.
Leftovers will keep in an airtight container in the refrigerator for 3 months and in the freezer for 1 month.