Preheat the oven to fan 180°C fan and line a 12-hole muffin tray with cases.
Chop the dark chocolate into chunks, roughly the size of a chocolate chip.
Mash the bananas in a large bowl using a fork, until smooth and creamy.
Add the almond milk, coconut oil, cacao powder, spelt flour, coconut sugar, bicarbonate of soda, baking powder and salt, then stir until a smooth batter forms.
Stir the dark chocolate into the mixture. I always keep a small handful of it back so that I can put it on top of the muffins, as it makes them look even more chocolatey and delicious.
Spoon two tablespoons of mixture into each muffin case, scatter over some walnuts, if using, and add a little piece of chocolate to the top.
Bake for 25 minutes, then leave the muffins to cool for 10–15 minutes before serving.