Whisk together the hot sauce, 2 tablespoons grapeseed oil, the cayenne and 1/4 teaspoon salt in a small saucepan. Heat over low heat until ready to coat the cauliflower.
Heat 1 inch of oil in a large heavy deep skillet over medium-high heat to 375 degrees F.
Meanwhile, place the arrowroot in a large bowl. Season the arrowroot and cauliflower with salt and black pepper. Working in 3 batches, moisten the cauliflower with a bit of water, dredge in the arrowroot, shaking off the excess, then carefully lower into the oil. Fry the cauliflower, turning occasionally, until cooked through and the crust is golden brown and very crisp, 5 to 7 minutes. Transfer to a wire rack set inside a rimmed baking sheet and keep warm in the oven while you fry the remaining cauliflower.
Transfer the fried cauliflower to a large bowl, add the warmed Buffalo sauce and toss to coat. Serve immediately with celery sticks and ranch dressing, for dipping.
Whisk together all of the ingredients in a small bowl. Add salt and pepper to taste. Serve right away or refrigerate in an airtight container for up to 1 week.
The Buffalo sauce can be made ahead. Let it cool completely then refrigerate in an airtight container for up to 1 week. Rewarm before using.