Nikuman (Steamed Beef Buns)

Ingredients

• 1 cup milk
• 1/2 cup white sugar
• 1 (16-ounce bag) steamed bun flour
• 1/2 teaspoon salt
• 1 tablespoon olive oil
• 1 pound ground beef
• 1/2 cup diced shiitake mushroom caps
• 1 stalk green onion, diced
• 1 tablespoon cornstarch
• 1 teaspoon white sugar
• 1 teaspoon soy sauce
• 1 teaspoon oyster sauce
• 1 teaspoon sesame oil
• 1/8 teaspoon Chinese five-spice powder
• 1 pinch ground black pepper