Preheat oven to 350℉ and line a cupcake pan with liners.
In a cup or small bowl, combine the milk and vinegar. set aside and allow to rest for at least 10 minutes.
In a medium bowl combine the flour, cocoa powder, cinnamon, baking soda, and salt. Set aside.
In an electric mixer, cream together the butter and sugar for 5 minutes, scraping the bowl halfway through.
Add in the eggs one at a time, followed by the vanilla extract.
Carefully mix in the peanut butter.
Then alternating between the flour mixture and milk, mix into the bowl on low speed. Start with adding in the flour, then milk, followed by more flour, then milk. Continue the process until all the ingredients are used, ending with the milk.
Add in the mashed bananas and crushed nus (if using) just until well incorporated. Do not overmix.
Scoop the batter and fill the cupcake liners ¾ of the way. Top each muffin with chocolate chips.
Bake for 18-20 minutes, or until the center comes out clean with a toothpick. Remove from the oven and allow to cool for 5-10 minutes.
Notes
To Store: Seal any leftovers in an airtight container and refrigerate for up to 5 days. To freeze, seal each muffin in foil and seal in an freezer bag. Freeze for up to 6 months.