In a medium bowl, whisk the soy sauce, sesame oil, rice vinegar, honey, hot sauce, kosher salt, black pepper, and garlic powder. Pour over the oxtails and stir to coat.
Cover and cook on LOW for 8 hours, until the meat is very tender.
Temporarily transfer the cooked oxtails to a platter. Strain the liquids from the slow cooker into a measuring cup. Return the oxtails to the slow cooker, cover with the lid, and set to WARM.
Skim the fat layer off the top of the cooking liquids. Pour them into a medium saucepan. Heat the cooking liquids over medium heat. Whisk the cornstarch with a tablespoon of cold water and stir it into the saucepan. Keep cooking the sauce, whisking often, until it has thickened into a rich glaze, 3-5 minutes.
Remove the oxtails from the slow cooker onto a serving platter. Top them with the sauce and serve.
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Notes
This recipe yields about 20 ounces of cooked meat without bones.
If too much fat remains in the cooking liquids, the glaze will separate when you try to reduce it. Sometimes, I place the cup with cooking liquids in the freezer for a few minutes. It enables the fat to solidify somewhat, and then it's easier to remove.
The leftovers keep well in the fridge, in a sealed container, for 3-4 days. In fact, like many slow-cooked stews, they taste better the next day since storing allows the flavors to meld. I reheat them gently in the microwave, covered, at 50% power. You can freeze the leftovers for up to three months.