Clean the corn: Remove the husks and as much corn silk as possible. Rinse the cobs under cool water and rub away any remaining silk. Trim longer cobs to approximately 6 inches if needed.
Steam the corn: Add 1¼ cups room-temperature water to a pot fitted with a steamer basket. Add the corn and cover the pot. Bring the water to a boil over medium-high heat, about 3 minutes. Reduce the heat to medium and steam for 10 minutes.
Make the brushing sauce: While the corn steams, combine the garlic, shacha sauce, soy sauce, white pepper, sesame paste, and sugar, if using. Stir in ½ tablespoon water. The sauce should be thick but easy to brush. Add the remaining ½ tablespoon water only if needed. If the shacha is very dry, stir in ½ tablespoon neutral-flavored oil.
Brush the first side: Line the air fryer basket with parchment paper. Transfer the hot corn to the basket in one layer. Brush the sauce over only the exposed top of each cob.
Air fry: Air fry at 360°F (182°C) for 5 minutes without preheating.
Flip and brush again: Turn the corn so the uncoated side faces up. Brush with the remaining sauce and air fry at 360°F (182°C) for another 5 minutes.
Serve: Serve the corn warm.
Notes
Waxy corn: For white, waxy, or multicolored Taiwanese- or Korean-style corn, start with at least 1.5 cups of room-temperature water and steam for 15 minutes after the water boils.
Sauce texture: Shacha is textured rather than smooth. A few seasoned bits may crumble onto the parchment, and that is normal.
Why no added oil: Shacha already contains oil. Add the optional neutral oil only when using the dry sauce from the bottom of the jar.
Homemade sauce: Homemade shacha adds 25 minutes if you do not already have it prepared. It keeps refrigerated for 2 to 3 weeks.
Storage and reheating: Refrigerate leftovers for up to 5 days. Microwave at medium power in 30-second increments. This keeps the kernels juicy, although the exterior will be less crisp.