In a large bowl, using an electric mixer, mix boxed cake mix, water, oil, eggs, baking soda, and baking powder on medium speed for 2 minutes. The mixture should be combined and smooth. Add bananas; mix until combined. Gently mix in mini chocolate chips.
Scoop batter into cupcake liners, no more than ⅔ full. Bake for about 12-15 minutes. Check cupcakes around 13 minutes to avoid over-baking. Insert a toothpick into the center of the cupcake; if it comes out clean, it is done. Remove cupcakes from the oven and let cool in the pan for 4 minutes; transfer to a wire rack and cool completely. Cupcakes should be cooled completely before frosting.
For Chocolate Buttercream
In a large bowl using an electric mixer, cream butter and shortening until creamy and smooth. Add cocoa powder, vanilla extract, and salt; mix until combined.
Slowly add powdered sugar 1 cup at a time, beating well after each addition. Scrape down the bowl as necessary.
Once all powdered sugar has been mixed in, add 1 tablespoon of milk at a time and mix. Continue to add and mix until the desired consistency is achieved.
Frost using a decorating tip or spatula onto cooled cupcakes. Eat & Enjoy!
Notes
Store Banana Chocolate Chip Cupcakes in an airtight container. Cupcakes will keep for 4+ days.Store Chocolate American Buttercream in an airtight container in the refrigerator for 1-2 weeks. When you are ready to use, bring to room temperature and re-whip.