Preheat your oven to 200C / 180C fan / Gas Mark 4.
Line a muffin tin with muffin cases or cupcake cases.
Beat together margarine and sugar until fluffy.
Add vanilla extract and mix in.
Mix in the mashed bananas.
Sift in the flour and bicarbonate of soda.
Mix until combined fully.
Divide the mixture between the muffin cases.
Bake for around 15 minutes, until an inserted skewer or knife comes out clean.
Leave to cool.
For the icing, combine the margarine, icing sugar and peanut butter until fully mixed.
If the mixture is too stiff, add some plant-based milk to loosen it.
Pipe a generous swirl of buttercream on the top of each cupcake.
Place a banana chip in the buttercream as a final touch.