...I've used the recipe below and it's very good, I got the recipe from grouprecipes.com, thought I'd share it
Lemon Buttercream Frosting
3 1/2 cups confectioners sugar
1/2 cup of butter (1 stick) softened
1/4 cup of milk
1 t vanilla
1 T lemon zest (the rind of a lemon)
2 T lemon juice
How to make it:
mix the butter, lemon zest and vanilla together, making sure all the lumps are gone.
add the milk, and quickly cream it into the butter mixture.
by the 1/2 cup, mix in the sugar.
once the sugar is all in the bowl, cream on "high" for two minutes. (I use this time to clean up)
mix in lemon juice.
spread heartily and enjoy!