In a food processor or a blender combine the frozen bananas and the cocoa powder.
Process at the highest power for about 10 seconds. The mixture will resemble sand and might stop moving. Stop the processor and mix with a spoon.
Continue processing for 1-2 minutes, until the mixture becomes smooth and creamy. If necessary, stop again the processor and mix 1-2 more times.
Serve immediately. You can garnish your nice cream with fresh fruit, chocolate shaves, nuts or just eat it plain, it is always good!
Notes
I find the easiest way to freeze bananas is in plastic containers. That way they don't occupy much space in the freezer and you can have 2-3 containers with frozen bananas at all times.
Make sure your bananas have been in the freezer for at least 24 h before making the nice cream. You can keep them frozen for up to 2 months, afterwards they will become brown.