Preheat oven to 400 degrees. Place cauliflower florets in a large bowl.
In a small saucepan, combine the soy sauce, brown sugar, sriracha, vinegar and garlic powder. Bring to a simmer over medium-high heat, stirring often. Add the dissolved cornstarch and continue mixing until the sauce thickens, about 1-2 minutes.
Pour sauce over cauliflower florets and stir to evenly coat.
Lightly coat a large baking sheet with olive oil. Spread cauliflower on baking sheet so florets are not touching; this will help them crisp.
Bake 25-30 minutes until cauliflower is fork-tender. Top with optional sesame seeds and scallions and serve.