Add oil to a large skillet and place over medium-high heat. Once hot, add ground beef and cook until browned. Drain out the excess fat, place in a bowl and set aside.
Add butter to the skillet and place on medium heat. Once butter is melted, add onions and peppers and cook until softened, about 2 minutes. Add garlic and cook until fragrant, about 30 seconds.
Add beef back to skillet, along with tomato paste. Mix together. Add in ketchup, water, brown sugar, mustard, Worcestershire sauce, chili powder, salt, and pepper. Mix together to combine.
Cook mixture for 10 minutes, or until mixture has thickened. Remove from heat. Scoop about 2/3 cup into split potato buns and close to make a sandwich. Serve immediately.