Preheat the oven to 180C/350F/Gas Mark 4, lightly grease and line the bottom of your baking tin with baking paper.
In a large bowl mix together flour, cocoa/cacao powder, baking soda, baking powder and salt. I like to use a whisk but you could sieve all the ingredients into the bowl first.
Peel and mash the bananas.
Add apple cider vinegar to soya milk and stir it in. Vinegar will curdle the milk which is what's supposed to happen. It's your soya "buttermilk" that will help the cake rise and be light and fluffy.
Gently melt the chocolate in a small saucepan (you could also do it in a microwave or in a bowl set over a small pan with gently simmering water).
Once melted add sugar, oil, soya milk and vinegar mixture and vanilla extract (if using) and give it all a good mix until well combined.
Once combined mix in mashed bananas.
Next, add chocolate banana mixture into the dry flour mix and fold it gently until there are no dry flour bits left. Take care not to overmix. The batter will be pretty lumpy because of mashed bananas which is how it's supposed to be.
Pour the cake batter into prepared baking tin and bake in preheated oven for approximately 30 minutes until the skewer inserted into centre comes out clean.
Let the cake sit in a tin for 10 minutes before removing it onto cooling rack to cool completely.