Peel the bananas, wrap in foil, or place in a sealed container and freeze overnight. Put the cranberries in a sealed container and freeze for at least an hour or overnight as well.
Remove frozen bananas and cranberries from the freezer and let sit for about 30 minutes. Place the cranberries, vanilla bean seeds, sugar, and a tiny pinch of salt in a food processor or blender. Process until the cranberries are coarsely ground, using a little water (a tablespoon or two) if needed to help things blend.
Add the bananas to the processor and blend just until smooth and incorporated. Pour the ice cream into a container with a lid and place in the freezer to set, about 3 hours.
Remove from freezer a few minutes before serving. Scoop into individual bowls and garnish as desired.