Preheat oven to 325°F.  Line or grease a muffin pan.

Tear the kale leaves into bite-sized pieces and boil or steam for a few minutes until tender. Refresh in cold water, drain and puree in a food processor or blender (it will still be a bit stringy). 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, zest and lemon juice.

Sift in the flour, baking powder and salt and stir to gently combine.

Fill the muffin cups ¾ full and bake for 15-20 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.