Cut the potatoes in half lengthwise and with a teaspoon carefully scoop out some of the warm potato into a medium bowl, leaving a 1/2-inch-thick shell of sweet potato. The potatoes should look like little canoes. Sprinkle the potato shells generously with salt and pepper. Add the remaining 4 tablespoons of butter to the scooped-out potato flesh and mash it with a fork. Add the egg, shallots, Taleggio cheese, 3/4 teaspoon salt, and 1/4 teaspoon pepper and stir until combined. Spoon the mixture into the sweet potato shells and top each with a sprig of fresh thyme. Bake for 20 to 30 minutes, until puffed and lightly browned. Serve hot.