Combine the butter and cocoa powder in a large, heavy-bottomed pot over medium-high heat, stirring to combine as the butter melts completely. Add the marshmallows and let them soften and melt; stir to keep from them from sticking, if necessary.
Remove the pot from the heat and add the crisped rice cereal, stirring until it is well coated with the marshmallow mixture. Add half of the chocolate morsels; mix well. Let the mixture cool for about 5 minutes, then add the remaining chocolate morsels. Let the mixture cool to room temperature.
Meanwhile, line a rimmed baking sheet with parchment paper or wax paper. Lightly grease your hands with nonstick cooking oil spray.
Use your hands to shape some of the coated cereal mixture into a golfball-size ball. Place on the baking sheet and repeat with the remaining mixture.
When the balls have set (feeling slightly dry to the touch), store them in an airtight container.
Adapted from his "Chocolate Epiphany" (Clarkson Potter, May 2008).