Preheat your oven to 425°F (220°C). Line a large rimmed baking sheet with foil or parchment paper.
In a medium bowl, whisk together the Dijon mustard, maple syrup, apple cider vinegar, chopped rosemary, salt, and pepper until smooth.
Pat the chicken drumsticks dry with paper towels. This helps the sauce adhere and the skin to brown.
Reserve about half of the maple Dijon sauce in a small bowl for basting later.
Place the drumsticks in a large bowl or a resealable plastic bag. Pour the remaining sauce over the chicken and toss until every piece is evenly coated.
Arrange the coated drumsticks on the prepared baking sheet in a single layer, leaving a little space between each one.
Bake for 20 minutes. Remove the baking sheet from the oven, flip the drumsticks, and brush them generously with the reserved sauce.
Return to the oven and bake for another 15-20 minutes, or until the chicken is cooked through and the glaze is caramelized. The internal temperature must reach 165°F (74°C) on a meat thermometer inserted into the thickest part without touching bone.
Let the chicken rest on the baking sheet for 5 minutes before serving to allow the juices to redistribute.