In a large bowl, cream together butter and sugar. Add eggs one at a time, beating well after each addition. Beat in bananas and Nutella.
In a second bowl, sift together baking soda, baking powder, and flour. Add dry ingredients to butter mixture by batches, alternating with the milk, until everything is combined.
Grease a muffin tin, or use paper liners. Portion the batter into the muffin tin. Bake in a preheated 350F oven for 25-30 min or until a toothpick inserted in one muffin comes out dry and almost clean. Set tin on a baking rack for 15 minutes, then turn muffins out and leave on cooling rack until fully cooled.
Meanwhile, melt chocolate and butter in the top of a double boiler. When muffins are cool, spoon chocolate over the top of each muffin. When chocolate cools and sets, wrap each muffin and store in refrigerator.