In a medium saucepan melt butter. Add mushrooms and saute till tender. Remove mushrooms, reserving drippings in pan. Stir flour, salt and pepper into drippings till smooth. Gradually stir in milk; cook over medium-high heat till thickened and bubbly, stirring constantly. Remove from heat; add cheese, stirring till melted. Stir in mushrooms and ham.

Beat egg whites and cream of tartar at high speed of electric mixer until stiff peaks form. Using clean beaters, beat egg yolks till thickened and lemon-colored, about 5 minutes. Fold ham mixture into yolks. Pour the yolk mixture over beaten egg whites; fold gently. Spoon mixture into 6 ungreased 1 1/2-cup souffle dishes. Bake in a 300 degree oven for 45-50 minutes or till a knife inserted near center comes out clean. Serve immediately.