Bake the whole sweet potatoes: 1. Preheat to 375°F. 2. Place the sweet potatoes aluminum foil covered baking sheet. 3. Bake until tender, about 1 hour.

Cool the baked sweet potatoes until you can comfortably handle them, about 20 minutes. Use a spoon to scoop the cooked insides out of the sweet potato peel. (You can do this step ahead of time and store in the refrigerator for up to 3 days before proceeding.)

Generously coat a 3-quart baking dish with the 1 tablespoon room-temperature butter.

Place the cooked sweet potatoes, 4 tablespoons melted butter, brown sugar, salt, cinnamon, nutmeg, and fresh thyme leaves in a large bowl. Mash until combined and mostly smooth. Add 4 egg yolks and stir until combined.

Transfer the mashed sweet potato mixture into the prepared baking dish and bake for 30 minutes.

To prepare the meringue: Beat egg whites, cream of tartar, and salt on low speed for 30 seconds. Increase the speed to high and beat for about 1 minute, until soft peaks form. Gradually add granulated sugar a little at a time, beating until firm, glossy peaks form, about 5 minutes.

Remove baked sweet potatoes and spread meringue on top. Fluff the meringue with the back of a spoon to make decorative peaks. Return to oven for 10-15 minutes, until the meringue is browned.