Oven 

1. To make this in the oven, preheat the oven to 160°C fan/180°C/Gas Mark 4. Line a 900g loaf tin (pan) with non-stick baking parchment. 
2. In a large bowl, cream together the butter and sugar until light and fluffy (I prefer to use an electric hand mixer for this). Add the beaten eggs and mashed banana and mix until well combined. 
3. Sift in the flour, cocoa powder, xanthan gum and bicarbonate of soda and mix briefly until no dry flour can be seen. Lastly, add the chocolate chips, if using, and mix once more. 
4. Spoon the mixture into the prepared tin and bake in the oven for about 1 hour.  
5. Check that it’s cooked by sticking a skewer into the centre – if it comes out clean, then it’s done. Allow to cool in the tin briefly, then carefully lift onto a wire rack to cool. 

Air fryer 

1. To make this in an air fryer, heat or preheat the air fryer to 150°C. Make and spoon the mixture into the prepared tin as opposite and air fry for 40–50 minutes – check with a skewer as opposite to ensure it’s done. 
2. Meanwhile, to make the frosting, melt the chocolate (I do this in the microwave, stirring between short bursts until melted), then allow to cool slightly. 
3. Place the butter in a large bowl and mix for 5 minutes with an electric hand mixer until the butter is fluffy and pale. Add the icing sugar and start mixing slowly to avoid creating a mini icing sugar explosion but then increase the speed as it starts to combine. Sift in the cocoa powder then mix again. 
4. Add the slightly cooled, melted chocolate to the bowl and mix until fully incorporated. If it seems too thick, add a teaspoon of milk to loosen it up. 
5.  Spread over the top of the cooled banana cake and finish with extra chocolate chips, if you like.