In large bowl, combine flour, sugar, baking soda, salt and baking powder.
In second large bowl, whisk eggs and pureed avocado until combined; whisk in oil, milk, lemon, lime & orange juices, and 1 tablespoon each of the orange & lemon zests; stir into flour mixture, just until combined. Pour batter into prepared pan.
Bake 18-20 minutes or until toothpick inserted in center of cupcake comes out clean. Cool in pan 5 minutes. Remove to cooling grid and cool completely.
For icing, combine buttercream with remaining 2 tablespoons orange zest. Ice cupcakes; top with avocado and orange zest.