Bring butter and water to a boil in a small saucepan.
While mixture is heating, combine dry ingredients in a large mixing bowl.
Pour boiling mixture over the dry ingredients and beat till smooth. Add egg, sour cream, lime juice, and zest and beat again.
Fill paper lined muffin cups 2/3 full. Bake at 350 for about 15-18 minutes.
Cool completely and frost.
For frosting: Beat butter and cream cheese till fluffy. Add remaining ingredients, using enough lime juice to achieve the consistency you want. You need it thicker for piping, thinner if you will just be spreading it on.