Remove frozen banana from freezer.
Either set on counter for 15 minutes to soften slightly or place in microwave for 5 seconds.
Place slightly defrosted banana and berries in food processor.
Close lid, leaving feed tube open.
Blend on high, adding 1/3 cup milk shortly thereafter.
After 10 seconds, stop processing. Remove lid, scrape down sides, and add peanut butter powder and flax seed.
Blend on high, adding more milk to get the desired consistency.
Transfer to serving bowl.
Serve with granola or other cereal to add texture, if desired.