In a small bowl, combine 4 tbsp milk and 1 tsp apple cider vinegar. Stir and set aside for 5 minutes to make buttermilk.
In a large mixing bowl, mash the overripe bananas well with a fork until smooth.
Add buttermilk, oil, vanilla extract, almond extract, and brown sugar to the mashed bananas. Whisk until well combined.
Add melted butter and whisk again until fully incorporated.
Sift in the all-purpose flour. Add baking powder, baking soda, cinnamon powder, and allspice.
Fold gently with a spatula until just combined. Do not overmix.
Fold in the chocolate chips.
Line a muffin tray with 6 paper liners. Fill each liner with 2 scoops of batter.
Top each muffin generously with dark chocolate callets.
Bake in a preheated oven at 180°C for 35–40 minutes, or until a toothpick inserted in the centre comes out clean.
Allow to rest in the tray for 10 minutes before removing. Enjoy warm!