Preheat oven to 170C/325F. Line or grease a muffin tray.
Tear kale leaves into bite-sized pieces and boil or steam for a few minutes until tender. Refresh in cold water, drain well and puree it with the orange juice with a hand held stick blender. The orange juice should make it easier to puree, but it will still be a little bit stringy. The more pureed you get it the greener the cupcakes will be. Set aside.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs, one at a time, and then beat in the kale, vanilla and orange zest.
Sift in the flour, baking powder and salt and stir to gently combine.
Fill the muffin cups ¾ full with the batter and bake for 20 minutes or until an inserted skewer comes out clean. Allow to cool in the tins for 10 minutes then remove and allow to cool completely on a wire rack. Frost with the buttercream when cold.
For the icing:
In a large bowl, cream the butter until fluffy. Sift in the icing sugar and beat until thick. Add enough orange juice to make it a frosting consistency. Store in the fridge until ready to use.