Cut the burger buns into two pieces horizontally with bread knife. Spread butter on the white sides.
Toast the buttered side buns on a pan for few minutes until golden brown and crispy. Set aside.
In a pan on medium heat combine cooking oil and onion. Sauté for few minutes until the onion is translucent.
Then add garlic and stir for 1 minutes.
Add tomato paste, saute and stir for about 2 minutes so it's caramelized and its sourness reduced.
After that add ground beef and sauté until all the beef is cooked and a little bit charred. Add salt to your taste.
For the seasoning add beef stock powder, chili powder, sugar, black pepper, worcestershire sauce, and Indonesian chili sauce. Stir until all combine. Add salt if necessary.
Put about 2-3 tbsp sloppy joe on each bottom half of bun. Add handful of mozzarella cheese on top. Broil until the mozzarella cheese has melted. Then put the top half of bun on top.
Cheesy sloppy joe burgers are ready to serve.