Combine first 3 ingredients in a medium bowl; set aside.
Beat egg yolk; add milk, lemon rind, and juice. Add to dry ingredients, and mix well. Beat egg white (at room temperature) until soft peaks form; fold into milk mixture.
Pour into 2 lightly greased 10-ounce custard cups; place in a pan of warm water. Bake at 350°F for 30 to 35 minutes or until a knife inserted in center comes out clean. Serve warm; top with whipped cream, if desired. Yield: 2 servings.