In a mixing bowl, cream butter and sugar. Beat in eggs, one at a time. Add lemon peel and vanilla; mix well. Combine dry ingredients; add to creamed mixture alternately with sour cream (batter will be thick). Fill greased or paper-lined muffin cups with quarter cup of batter. Bake at 350� for 25-30 minutes or until a toothpick inserted near the center comes out clean. Cool for 10 minutes; remove to wire racks to cool completely. For frosting, cream butter and sugar in small mixing bowl. Add lemon juice, vanilla, lemon peel and milk; beat until smooth. Frost cupcakes.