Set frozen bananas out for 5 minutes to thaw slightly. Place them in a high-speed blender (preferably a Vitamix) with nut butter and cocoa powder. Blend on medium sped then turn the blender up to the highest setting and continue to blend until it reaches a smooth consistency, 1-2 minutes. The Vitamix has an internal fan that switches on at the highest setting, so turn it all the way up.
4 frozen bananas, 1/4 cup cocoa powder, 2 Tablespoons nut butter