Preheat the oven to 180ºC. Line a cake / muffin baking tray with 12 muffin cases.
Mash the bananas well and add the mashed banana to a large mixing bowl along with the yoghurt, eggs, sugar, melted butter, and vanilla. Whisk the ingredients together well.
In a separate bowl, stir together dry ingredients. So that's the flour, cocoa powder, baking powder, baking soda, and salt.
Pour this into the bowl with the wet ingredients and stir until they are combined. Be sure not to over mix, It's okay if the mixture has a few bumps.
Next, fold the chocolate chips into the mixture.
Divide between the 12 muffin cases.
Transfer to the preheated oven and bake for 35 minutes.
After about 35 minutes, remove the muffins from the tin and allow them to cool on a wire rack before serving.