Pierce the sweet potatoes and microwave on a microwave-safe plate for 10 to 14 minutes, turning halfway, until completely soft. Let stand for 5 minutes, then scoop out the flesh.
Heat the oven to 375°F and grease six 6-ounce ramekins. Mash the potato flesh with the 2 tablespoons butter, milk, honey, salt and nutmeg; cool until warm, then mix in the egg yolks.
Beat the egg whites in a clean bowl until soft peaks hold briefly. Fold one-third into the potatoes to loosen them, then gently fold in the rest.
Divide among the ramekins, filling no higher than three-quarters full. Toss panko with the topping butter and scatter a thin layer over each portion.
Set the ramekins on a baking sheet and bake for 22 to 26 minutes until lightly puffed, golden and 160°F in the centers. Let stand for 5 minutes before serving.