These should really be called 'leftovers' cupcakes, as the idea was conceived when I couldn't initially find a pan big enough to bake the Rhubarb Upside-Down Cake in. "Don't worry" I said to myself, "CT doesn't like rhubarb anyway, so you'll have to bake him something - cook the excess batter as cupcakes".
Then I found a skillet type pan which was big enough to take all the rhubarb cake batter. But the need to bake for CT didn't go away and there were still leftovers from the cake - a small quantity of sour cream and the zest of half an orange, plus I really liked the idea of a crumble topping on cupcakes. So I made these: