1. Preheat the oven to 180° C (350° F). 
2. In a bowl, cream the butter and sugar; add the grated lemon zest. 
3. Blend in the eggs, one at a time, then the sour cream and lemon juice. 
4. In another bowl, combine the dry ingredients (flour, baking powder, baking soda and salt). Incorporate the dry ingredients into the butter mixture (step 3), being careful not to overmix.
5. Line muffin tins with paper liners; fill three-quarters full with batter. 
6. Bake for about 18-20 minutes - the cupcakes are done when a toothpick or the tip of a knife inserted into the center comes out clean. Cool the cupcakes.

Frosting

1. Beat an egg white to stiff peaks. 
2. Cream together the cream cheese and icing sugar; add the lemon juice. 
3. Gently fold in the beaten egg white to lighten. 
4. Ice the top of each cupcake with frosting, using a knife or a piping bag. 
5. Decorate with a little candied lemon peel.