Heat oven to 170C/150C fan/gas 31⁄2. Line muffin tins with 16 cases. In a bowl, combine the flour, sugar, 1⁄2 tsp salt and bicarbonate of soda. In a second bowl or a jug, mix the mayonnaise, mashed bananas and vanilla extract. Pour the wet ingredients into the dry and mix with a spoon until just combined (don’t overmix or your cupcakes will be heavy). Spoon the mixture into the cases and bake for 20 mins.
When the cupcakes come out of the oven, sprinkle the chocolate chips over – they will melt and then harden again, so don’t touch them.
For the icing, combine the vegan margarine and icing sugar in an electric mixer, then add the vegan milk and continue to mix on a slow speed until completely combined. Turn the mixer up and combine for a further 3 mins. Finally, stir in the peanut butter. Pipe or simply spread the icing on top of the cakes. Store in an airtight container and eat within 2 days.