PreparationFor topping: Toss first 4 ingredients in medium bowl. Add butter; using fingertips, rub in until well blended. Cover; chill. DO AHEAD: Can be made 1 day ahead. Keep chilled. For pudding: 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 dish. Sprinkle with topping. Bake pudding until puffed and brown, about 45 minutes. Serve immediately.
PreparationFor topping: Toss first 4 ingredients in medium bowl. Add butter; using fingertips, rub in until well blended. Cover; chill. DO AHEAD: Can be made 1 day ahead. Keep chilled. For pudding: 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 dish. Sprinkle with topping. Bake pudding until puffed and brown, about 45 minutes. Serve immediately.