In a medium bowl, whisk together the mascarpone, cinnamon and vanilla until combined. Cover and refrigerate for at least 30 minutes to allow the flavors to meld.
Preheat the oven to 350 degrees F. Brush a baking dish with 1 tablespoon of the butter. Slice a little bit of the flesh from the bottom of each peach half so they sit evenly in the baking dish. Scoop a tiny bit from the center, so the pit space is a little larger. Put the peaches in the dish, cut-side up.
Whisk together the remaining 4 tablespoons butter, honey and black pepper in a small bowl and pour it evenly over the peaches. Roast the peaches until lightly golden brown and just soft, about 25 minutes.
Remove to a platter and top each half with a dollop of the cinnamon mascarpone.