4 sweet potatoes, about 2 pounds
Kosher salt and freshly ground black pepper
4 tablespoons (1/2 stick) tempered unsalted butter
¼ teaspoon cayenne, plus more as needed
½ vanilla bean, split lengthwise (seeds only)
Preheat the oven to 350°F. Puncture each sweet potato all over with a fork three times on each side. Rub with grapeseed oil. Season with salt and pepper. Wrap each individually with foil.
Set potatoes on a baking sheet and roast until they are noticeably soft and there is caramelized sugar forming around the puncture holes. At this point they can be easily pierced with a cake tester or fork, about 1 hour.
Carefully remove potatoes from foil and let them cool enough so that you can handle them. Peel the potatoes with a paring knife and discard the peels. Note: you can wear rubber gloves if it makes it easier to handle.
Place the peeled potatoes in a food processor with the tempered butter and cayenne.
Scrape the seeds of the vanilla bean into the potatoes. Purée until smooth. Season with salt, pepper, and more cayenne, as needed.