Place turkey and half of the sage in a resealable plastic bag. Separately whisk buttermilk with bouillon; add to the bag. Seal bag and place in baking dish or bowl. Refrigerate for 8 hours or overnight.
Preheat oven to 400˚ F. Remove turkey from buttermilk marinade; discard leftover marinade. Place turkey breast on rack in small roasting pan. Chop remaining sage leaves and combine with butter; rub all over turkey breast.
Bake for 45 to 60 minutes or until golden brown and juices run clear. Let stand for 15 minutes.