Preheat the oven to 350 degrees.
Line a muffin pan with muffin liners. In a mixing bowl, beat the bananas until mashed.
Next, add the melted coconut oil, coconut sugar, and egg.
Beat the wet ingredients until combined.
Add the cocoa powder, buckwheat flour, espresso powder, and baking powder to the mixing bowl. Beat the dry ingredients into the wet ingredients.
Next, add the baking soda and vinegar to a small bowl. Stir to combine. Once settled, add the vinegar and vanilla to the mixing bowl.
Beat these ingredients into the muffin batter.
Lastly, fold in the chocolate chips.
Using a scoop, scoop the batter into the muffin liners so they are ¾ of the way full.
Once all 12 liners are filled, place the muffin pan into the preheated oven.
Bake the muffins for 15-20 minutes or until the inside of the muffin is baked through.
Repeat steps 9-11 with the remaining batter.
Store leftover muffins in an airtight container once completely cooled.