Melt butter; blend in flour. Heat milk, cocoa, and sugar together until hot, but not boiling; add all at once to flour mixture, stirring constantly. Beat egg yolks until light. Blend small amount of sauce into yolks; add mixture to sauce. Cook and stir over low heat until slightly thickened. Add vanilla. Cover and cool. Beat egg whites until stiff but not dry; fold into cooled chocolate mixture. Lay sliced pears in bottom of buttered 4-cup baking dish; pour chocolate mixture over. Bake at 350 degrees for 30 minutes or until set. Serve immediately.