Preheat the oven to 180ºc (160ºc fan) and line a 12 cup muffin tin with paper liners.
Start with the bananas, sugar and butter in a medium mixing bowl. Use an electric hand whisk here if you like. We're looking for the butter and sugar to be completely mixed and the banana to be broken down. It's perfect if there are still some chunks of banana in the mix.
Add in the flour, baking powder, eggs, milk and vinegar. Whisk everything well, but take care not to over mix.
Mix in the chocolate chips and use a large spoon or an ice cream scoop to drop the muffin mix into the muffin cases. I use about 4 tablespoons of the mix per case which ends up filling about ¾ of the muffin case. This makes 10 large and fluffy bakery-style muffins. If you prefer a smaller muffin then divide the batter between all 12 muffin cases.
This is totally optional, but if you have some demerara sugar, then may I suggest that you sprinkle some sugar onto the tops of the muffins before baking. It gives a super crunchy topping that just works stunningly with the chocolate in the muffins and brings out some extra banana flavour.