Finely chop half of the candied peel. Put the ricotta into a mixing bowl and use a wooden spoon to cream it until smooth. Add in the mascarpone, powdered sugar and chopped candied peel. Mix well then transfer to a piping bag fitted with a large star tip.
Slice the doughnuts in half across the middle and lay out the bottom halves on a baking tray. Pipe the ricotta filling in rounds on each of the donuts, then sprinkle over the remaining candied peel and chocolate chips. Top with the doughnut lids, and lightly dust with powdered sugar.