seeds scraped from 1 vanilla pod or 1 teaspoon vanilla bean paste
To make the icing, place the butter and icing sugar in a bowl and beat together with a hand-held electric whisk until light and fluffy. Add the mascarpone, lemon rind and vanilla seeds or paste and beat until smooth, taking care not to over-beat as the mixture will split.
Spoon the icing into a piping bag fitted with a star-shaped piping nozzle, then pipe over the cupcakes.
Decorate with the blueberries, then serve sprinkled with purple sugar sprinkles, if liked.