Mashed Potatoes with Ham

3 cups hot mashed potatoes
2 egg yolks
1 cup sour cream
2 cups diced cooked ham
2 egg whites
salt
white pepper
2 tablespoons butter
Preheat oven to 350°F.
Beat egg yolks with mashed potatoes and season with salt and pepper.
Add sour cream and ham.
Beat egg whites until stiff and fold into potato mixture.
Place in a buttered casserole, dot with butter and bake 30 minutes.