1. Place potatoes in shallow baking dish. Microwave until skewer glides easily through flesh and potatoes yield to gentle pressure, 9 to 12 minutes, flipping potatoes every 3 minutes. Let potatoes cool for 10 minutes. Adjust oven rack to middle position and heat oven to 425°F.

2. Combine sugar, cinnamon, cayenne, and pinch salt in medium bowl. Add bread pieces and toss to combine. Melt butter in 8-inch skillet over medium heat. Reduce heat to medium-low and continue to cook, swirling pan frequently, until foaming subsides and butter is just beginning to brown, 2 to 4 minutes. Reserve 2 tablespoons browned butter and pour remaining butter over bread pieces. Toss to combine.

3. Halve each potato lengthwise. Using spoon, scoop flesh from each half into second bowl, leaving about 1/8 to 1/4-inch thickness of flesh. Place 6 shells cut side up on wire rack set in rimmed baking sheet (discard remaining 2 shells). Bake shells until dry and slightly crispy, about 10 minutes.

4. Meanwhile, mash potato flesh with ricer, food mill, or potato masher until smooth. Stir in egg, 1/2 teaspoon salt, and reserved 2 tablespoons browned butter.

5. Remove shells from oven and reduce temperature to 375°F. Divide mashed potato mixture evenly among shells. Top each filled shell with bread mixture. Bake until bread mixture is spotty brown, about 20 minutes. Let cool for 5 minutes before serving.

TO MAKE AHEAD: Stuff potato shells as directed in step 5 but do not top. Store bread mixture at room temperature and refrigerate filled potatoes for up to 24 hours. To serve, top each shell and bake as directed, reducing oven temperature to 325°F and increasing baking time to 35 minutes.

Serves 6