Place the roast into a large resealable bag. In a small bowl, stir together the melted butter, vinegar, pancake syrup, cornstarch and brown sugar. Pour into the bag with the roast. Seal, and turn to coat. Refrigerate for 2 hours.
Preheat the oven to 350 degrees F (175 degrees C). Place the roast into a roasting pan or large Dutch oven, and pour the marinade over it.
Cover, and roast for 1 hour in the preheated oven. Remove the lid, and continue roasting until the top is browned and the internal temperature is at least 145 degrees F (63 degrees C) when taken with a meat thermometer. Remove from the oven, and let rest for about 10 minutes before carving. The internal temperature should increase by 10 degrees.