Heat oven to 170C/150C fan/gas mark 3. Line muffin tins with 16 cases.
In a bowl, combine the flour, sugar, 1⁄2 tsp salt and bicarbonate of soda. In another bowl 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.
Leave in the tray for 5 minutes then move to a cooling rack.
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.
For the fondant (optional). Use white ready to roll fondant icing then add whatever colour food colour gel you would like then fold the fondant over and work until the fondant is the colour you would like it to be. Then use cutters to get your decorations then add to the top of the cakes.