Preheat oven to 400°F.
Roast potatoes on baking sheet until tender, 45 to 60 minutes.
Cool.
Cut in half lengthwise. Scoop flesh into bowl; mash.
Transfer 4 cups potatoes to large bowl.
DO AHEAD Can be made 1 day ahead. Cover; chill.
Bring to room temperature before continuing.
Preheat oven to 350°F.
Butter 13x9x2-inch baking dish.
Add half and half and next 5 ingredients to potatoes.
Using mixer, beat on low speed until blended.
Season with salt and pepper.
Add yolks 1 at a time, blending after each addition.
Using clean dry beaters, beat whites in another large bowl until foamy.
Add salt and cream of tartar.
Beat until peaks form.
Fold whites into potatoes; spoon into baking dish.
Sprinkle with topping.
Bake pudding until puffed and brown, about 45 minutes.
Serve immediately.