Banana and chocolate muffins

Name: Banana and chocolate muffins
Ingredients: 75g/2¾oz unsalted butter; 275g/9¾oz plain flour; 1 tsp bicarbonate of soda; 2 tsp baking powder; ¼ tsp ground cinnamon; 120g/4¼oz golden caster sugar; 100g/3½oz dark or milk chocolate chips (or a mixture); ¼ tsp fine salt; 2 tbsp flavourless oil, such as sunflower or light rapeseed, plus extra for greasing; 100ml/3½fl oz milk; 1 large free-range egg; 1 tsp vanilla extract; 250g/9oz ripe banana (about 3 large bananas), mashed; seeds or chopped nuts, for decorating (optional)