Position an oven rack in the center of the oven, put a baking sheet on it and preheat to 450 degrees F. Cut the cauliflower through the core into 1-inch-thick steaks. It's fine if some of the end steaks break into florets; just cook them along with the rest.
Put 2 tablespoons of the butter on the hot baking sheet, return the sheet to the oven and let the butter melt and bubble, about 30 seconds. Remove the baking sheet from the oven and tilt to coat it with butter. Arrange the cauliflower steaks and any stray florets on the baking sheet. Roast until the bottom of the cauliflower is dark brown and almost tender (a knife inserted in the steaks should meet a little resistance), about 12 minutes. Generously sprinkle the cauliflower with salt, flip with a spatula, sprinkle with salt and continue to roast until the cauliflower is tender, about 10 minutes more.
While the cauliflower roasts, mash the remaining 4 tablespoons butter with a fork in a small bowl. Mash in the parsley, capers, caper brine, lemon zest, 1/4 teaspoon salt and one grind of pepper. When the steaks are done, transfer them to a serving platter or individual plates. Dollop some compound butter on top of each steak and scatter any stray florets around them. Serve hot.
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