For the frosting

1. Using a mixer, cream butter till pale.
2. Add in icing sugar, lemon juice, vanilla and continue beating till smooth and creamy.
3. If the consistency is too thick, add milk.
4. Mix in lemon zest.

For the cupcake

1. Preheat the oven to 350 degree F (180 C).
2. Line thirty 2 1/2-inch muffin cups with paper bake cups; set aside.
3. In a medium bowl, combine flour, baking powder, baking soda, and salt; set aside.
4. In a large bowl, beat butter with an electric mixer on medium to high speed for 30 seconds.
5. Add granulated sugar and vanilla; beat until well mixed.
6. Add eggs, one at a time, beating well after each addition.
7. Mix in flour mixture, sour cream and milk alternately to beaten mixture, beating on low speed after each addition just until combined.
8. Fold in lemon peel.
9. Spoon about 1/4 cup of the batter into each prepared muffin cup.
10. Bake in preheated oven about 18 minutes or until a wooden toothpick inserted in centers comes out clean.
11. Cool completely on a wire rack before frosting.