I first saw these at Esurientes who saw them at Baking Sheet who got them from Donna . Joey over at 80 Breakfasts also made them and hers look fantastic. She has managed to get perfect swirls of Nutella.
I skipped the "soften the Nutella" bit and just placed a spoonful on top of the cake mixture. It was impossible to swirl and the result is a flat cupcake with a blob of Nutella on the top. Apart from the swirl they were unbelievably easy and quick to make. Eat them fresh from the oven while they are still warm.
I'll make them again and this time I'll soften the Nutella first.