Green smoothie cupcakes use green smoothie as an ingredient in both the cupcake and the buttercream frosting. The green smoothie gives the cupcakes a mildly fruity flavor and a slightly denser than average texture.
I imagine that any smoothie recipe sans ice would work in these cupcakes. However, I haven't tried them all and I can only vouch for the one I provide below. My green smoothie recipe yields much more smoothie than you will need for the cupcakes. Go ahead and drink the extra smoothie while you are working.