Preheat the oven to 180C/350F/gas mark 4. Grease two 20cm/8in round cake tins and line the bases with baking parchment.
Whisk together the flour, bicarbonate of soda, cinnamon and salt on a low speed until well combined.
Whisk together the mashed banana, light brown soft sugar, sunflower oil, plant milk, white wine vinegar and vanilla extract.
Pour the wet ingredients into the dry and mix on a low speed until just combined. Be careful not to over-mix or the cake can become tough.
Divide the batter evenly between the tins and bake for 25-30 minutes until firm to the touch and a skewer inserted into the centre comes out clean.
Leave the cakes to cool in their tins for 10 minutes then carefully turn them out onto a wire rack and leave to cool completely.
To make the buttercream; melt the chocolate, either in a bowl over a pan of gently simmering water or in short bursts in the microwave, stirring regularly. Set aside to cool a little.
Place the margarine and vegetable shortening in the bowl of a stan mixer and whisk until smooth. Add the sifted icing sugar and mix on a low speed until combined then turn the mixer up to a high speed and whisk for a couple of minutes until fluffy.
Whisk in the cocoa powder and melted chocolate followed by the peanut butter, vanilla extract and salt. If it is too thick you can whisk in a drop of non-dairy milk.
Place one of the cooled cakes on a serving platter or cake stand and spread over some of the buttercream. Place the other cake layer on top and cover the top and sides of the cake with buttercream.
Decorate with banana chips and chocolate curls if desired. Store in an airtight container at cool room temperature for up to 5 days (keep in the fridge if it is hot out).