Combine tempura mix and water in a bowl; whisk until blended. Set bowl into a larger bowl filled with ice cubes to keep cold.
Mix hoisin sauce, 2 tablespoons sesame oil, vinegar, soy sauce, sesame seeds, and garlic powder together in a small bowl.
Dip cauliflower into the tempura batter and set on a plate to let excess batter drip off.
Combine vegetable oil and remaining 3 tablespoons sesame oil in a medium saucepan over medium heat; bring temperature up to 350 to 375 degrees F (175 to 190 degrees C).
Deep-fry cauliflower florets in batches until golden brown, 4 to 6 minutes per batch. Remove to a paper towel-lined plate to drain.
Serve with hoisin mixture drizzled on top, or on the side for dipping.