Peel, slice and freeze the bananas in an airtight container or bag for at least 4 hours or overnight. Once the bananas are frozen, place them in a food processor. Process until the bananas have the texture of lumpy oatmeal (about 1 minute). Add the coconut milk, green tea powder and vanilla, then process until very smooth and creamy, about another 2 minutes, scraping down the sides of the processor as needed.
Once the ice cream is completely smooth, scrape into an airtight container, cover and freeze for 1 to 2 hours for a soft-serve consistency. For a firmer, more scoop-friendly ice cream, freeze for over 4 hours and allow to temper at room temperature for 10 minutes before scooping.