Blend the kale, mint, milk and sugar until very smooth. Add the mint extract if using and yogurt and blend minimally until smooth.
Refrigerate for half an hour to aid freezing. Pour into an ice cream maker, churning according to manufacturers instructions until thick – about 15 minutes. Five minutes from the end, add the chopped chocolate.
How to make without an ice cream maker
If you don’t have an ice cream maker, freeze for an hour in a shallow freezer-safe dish, whiz in the blender and then freeze for another hour.