Preheat the oven to 325 degrees F (165 degrees C).

Place roast in a roasting pan; cover pan. Cook in the preheated oven until beef falls apart and shreds easily, about 2 hours.

Meanwhile, melt butter in a large skillet over medium heat. Add onion; cook until translucent.

Stir chili sauce and vinegar into onion. Fill empty chili sauce bottle with water, shake, and pour liquid into skillet. Stir in garlic, brown sugar, Worcestershire sauce, mustard powder, black pepper, salt, and cayenne pepper; cook over low heat, stirring often, until thickened.

Shred beef using 2 forks; stir into sauce and simmer for 30 minutes.