Combine egg yolks and cornstarch in a large heavy saucepan; beat well. Gradually add milk, stirring constantly. Cook over medium heat, stirring constantly with a metal spoon, 2 minutes or until mixture thickens and coats the spoon. Remove from heat; add butter, and stir until butter melts. Pour mixture into an 8-inch square baking dish.
Beat egg whites (at room temperature) until foamy. Gradually add sugar, 1 tablespoon at a time, until stiff peaks form. Spread meringue over hot pudding. Bake at 350° for 10 minutes or until lightly browned. Cool to room temperature. Spoon into individual serving bowls.