Roast the sweet potatoes. Preheat the oven to 400°F. Poke a few holes in each sweet potato with a fork. Bake for about 1 hour to 1 hour 20 minutes, always go a little longer, until very soft and tender. Let cool slightly, then scoop out the flesh, reserving the skins if desired.
Brown the butter. In a skillet over medium heat, brown the butter until golden and nutty. Add the fresh thyme and cook for 1–2 minutes, until crispy. Remove from the heat, reserving some of the brown butter and crispy thyme for serving
Whip the potatoes. Reduce the oven temperature to 350°F. Add the roasted sweet potatoes, maple syrup, vanilla, cinnamon, chipotle, cayenne, milk, bourbon, parmesan, and 5–6 tablespoons of brown butter to a food processor. Season with salt and pepper. Blend until smooth and creamy, then mix in the eggs. Alternatively, use an electric mixer.
Bake and serve. Spoon the whipped sweet potatoes into a buttered 9×13-inch baking dish or a similarly sized dish. Bake for 35–40 minutes, until set. Spoon the remaining warm brown butter and crispy thyme over the casserole. Finish with flaky sea salt and freshly cracked black pepper. Serve warm.