Start by picking a medium saucepan. Add the granulated sugar, dark brown sugar, soy sauce, sake, mirin, and rice wine vinegar. This foundation will become the syrupy goodness that defines our sauce. Turn on medium heat and stir until the sugars dissolve. You need a little patience here; don’t let it rush.

Once the base is ready, add minced garlic, grated ginger, black pepper, toasted sesame oil, and chili flakes to the saucepan. Stir these ingredients in well. The garlic and ginger will release their aromatic goodness and start to perfume your kitchen. This part excites me—just wait until you taste this blend.

In a small bowl, combine the tablespoon of cornstarch with a bit of water. This is your secret weapon for achieving that glossy, luscious consistency. Slowly add this mixture to your saucepan, stirring constantly to avoid lumps. As the sauce heats, it will thicken. Keep stirring until you achieve a syrupy texture. You’re on the right track now!

Once the sauce thickens to your liking, reduce the heat and let it simmer for a few minutes. This step allows all those flavors to meld into the divine creation you’ve been waiting for. Taste it along the way—what's your impression?

Remove the saucepan from heat and let your sauce cool completely. When ready, you can pour it into a jar for easy storage. Trust me; it’s hard to resist sneaking tastes during this stage!