Let bubble for 5 minutes without stirring

Sift the flour into a bowl and add the xanthan gum if using, baking powder, bicarb of soda and sugar

Make a well in the middle and add the milk, lemon juice, melted margarine, vanilla and mashed banana

Stir into a smooth batter

Pour evenly between the cupcake liners

Bake for 15 - 20 minutes until risen and firm to the touch

Meanwhile, to make the buttercream, whisk the softened margarine with the powdered sugar until light and creamy. Beat in the milk and vanilla extract. Add more icing sugar if the buttercream is too thin or more milk if it's too thick to pipe

Fill a piping bag (thin nozzle) with some of the butterscotch sauce and push the tip into top of each cupcake once cooled, applying enough pressure to fill each with some sauce

Fill another piping bag (wide nozzle) with the buttercream and swirl on generously

Drizzle with the remaining butterscotch sauce - the more the better!