Cream butter and sugar at medium speed of mixer until light and fluffy.
Add egg white and beat about 30 seconds.
Add sour cream, milk, vanilla, lime juice and zest and beat on low speed about 1 minute.
In a separate bowl, combine the dry ingredients.
Add dry ingredients to wet ingredients gradually, beating lightly after each addition.
Fill lined mini muffin tins ⅔ to ¾ full.
Bake at 375F degrees for about 10 minutes, or until cake springs back when lightly touched in center.
Cool on wire rack about 5 minutes and then remove from muffin tins to complete cooling.
Frost with Key Lime Whipped Cream.
Refrigerate until time to serve, and refrigerate any leftovers.