Preheat the oven to 350. Grease a 9×13-inch pan with cooking spray.
In a bowl, mix together the cake mix, bananas, eggs, milk, cinnamon and rum extract; the batter will be thick. Pour into the prepared pan and bake in the preheated oven for 28-32 minutes, or until a toothpick inserted in the center comes out clean. Cool completely.
While the cake is cooling, make the rum buttercream. Place the butter in the bowl of a stand mixer or in a large bowl. Beat until light and fluffy. Slowly beat in the sugar on low. Add the milk and the rum extract, and start beating on low. Slowly increase the speed and beat until the buttercream is creamy. If it is too thick, add more milk.
When the cake has cooled completely, crumble into a large bowl. Add the buttercream and mix with the back of a spoon until it is a thick dough consistency. Shape into balls and cool for 1-2 hours in the refrigerator or for 20 minutes in the freezer.
Dip in white chocolate, vanilla candy melts or almond bark.