In a medium bowl, cream together the butter and sugar until smooth. Mix in the egg and milk gradually, so as not to curdle the butter. Sift in the flour, and fold in gently. Transfer to a 1 pint microwave-safe bowl.
Cover, and cook for 3 1/2 minutes on full powder, or until the pudding appears set when gently jiggled, and the top is sticky. Serve hot.