Squeeze the juice from six large lemons or, if using them as cups, cut off their tops of the way down and carefully scoop out the flesh. (Put lemon cups in the freezer until ready to use.)
Put juice/flesh in a blender with a large handful of mint leaves and blend well. Strain juice through a sieve. Add simple syrup, vodka and limoncello and stir.
Pour into a container and put in the freezer. As it starts to freeze, use a fork to scrape the icicles. Do this every few hours. When fully frozen scrape again with a fork into icy piles and, if using lemon cups, spoon into them and serve.