Preheat the oven to 450 degrees F. Put the cauliflower florets on a rimmed baking sheet and drizzle with the vegetable oil. Toss well to coat, then spread the florets out into a single, even layer. Season liberally with salt. Roast the cauliflower until golden on the edges and tender throughout, about 24 minutes.
Put the lemon zest on a paper towel-lined microwave-safe plate and microwave in 30-second increments, stirring in between, until the zest is dry and almost brittle, 2 to 4 minutes total. (The timing will largely depend on the power of your microwave.)
Stir or whisk the zest with the buttermilk powder, chives, parsley, garlic salt, onion powder, black pepper, cayenne and 1/4 teaspoon kosher salt in a small bowl to combine.
Remove the cauliflower from the oven and, while still warm, toss with 3 tablespoons of the spice blend. (Reserve the remaining spice blend for another use. See Cook's Note below.) Transfer to a platter and serve.
The leftover spice blend will keep in an airtight container in the refrigerator for up to 1 week. Try it sprinkled on grilled chicken, warm buttered popcorn or warm flatbread; or stir it into sour cream and mayonnaise as a dip.
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