Stir all buttermilk marinade ingredients together in a large bowl, until combined.
Add cauliflower florets, stir well to coat thoroughly (let every nook and cranny get coated), set aside for about 15 minutes (make it right away, if pressed for time).
Into a large dutch oven or a large saucepan, pour vegetable oil until about 1-inch height, heat over high heat until moderately hot (about 350 degrees in a deep fry thermometer).
While the oil is heating, place all purpose flour and cornstarch in a shallow bowl, add remaining 1/4 teaspoon salt and 3 tablespoons of the prepared buttermilk marinade (try to scoop from the bottom of the bowl). Mix with your fingertips, until thoroughly combined (this creates texture in the flour and guaranteed crispiness).
Working with one floret at a time – remove from the buttermilk marinade, coat with flour (pressing on all sides for flour to adhere well) and place on a plate in a single layer.
Deep fry in three batches, until deep golden on all sides ( about 6 minutes per batch). Drain on a paper towel lined platter.
Serve hot for appetizer / snack / side dish with ketchup or ranch or Sriracha ranch.