Whisk the soy sauce, water, sugar, mirin, ginger, garlic, cornstarch, and chili-garlic sauce together in a small saucepan over medium heat. Simmer until the sugar dissolves and the sauce thickens enough to coat the back of a spoon, about 3 to 5 minutes, then remove from the heat.

Pat the tofu planks thoroughly dry with paper towels to ensure a clean sear. Brush both sides of each plank with vegetable oil and season with salt and pepper.

Prepare the grill for medium-high heat—the oil should shimmer but not smoke on the grates. Clean and oil the grill grate well to prevent the delicate tofu from sticking.

Place the tofu planks on the hot grill and cook until well-browned with distinct grill marks, about 5 to 7 minutes. Use two spatulas to carefully flip the planks for better leverage so they don't break.

Brush the top of each plank generously with about half of the prepared glaze. Continue to cook until the second side is well-browned, about 5 minutes.

Flip the tofu one last time and brush the second side with the remaining glaze. Cook for about 1 minute until the glaze is bubbly and sticky, then transfer to a platter.

Sprinkle with cilantro and serve immediately while the exterior is still crisp.