MAKE THE CAULIFLOWER:
In a medium bowl, whisk together the cornstarch and flour with 1 teaspoon kosher salt and 1/8 teaspoon pepper. Add the cauliflower pieces to the bowl, tossing to coat.
Add 2 to 3 inches of vegetable oil to a large heavy-bottomed stockpot set over medium-high heat. Line a plate with paper towels.
Once the oil is hot, shake off any excess flour mixture from the cauliflower pieces and add them to the oil in batches.
Cook the cauliflower in batches, turning as needed, until they golden brown. Using a slotted spoon, transfer the cooked cauliflower to the paper towel-lined plate then make the sauce.
MAKE THE SAUCE:
In a small bowl, whisk together the cornstarch and soy sauce to make a slurry. Set the slurry aside.
Add the vegetable to a large saucepan set over medium heat. Once the oil is hot, add the chilies, garlic and ginger and cook, stirring constantly, until the garlic is golden brown, about 1 minute.
Stir in the rice wine vinegar, hoisin, brown sugar, vegetable stock and sesame oil.
Stir in the slurry, bring the sauce to a simmer and cook it until it thickens to the consistency of syrup, about 1 minute.
Return the cauliflower to the pan and stir to coat then serve.