Heat a pan over medium heat and melt the butter.
Add the chopped onion and garlic, then sauté until fragrant and translucent.
Stir in the freshly ground black pepper, sugar, soy sauce, dark soy sauce, ketchup, milk, and water. Let it simmer for a couple of minutes.
Mix the cornstarch with water to make a slurry, then slowly pour it into the sauce while stirring.
Keep stirring until the sauce thickens and turns glossy. Remove from heat and set aside.