Preheat the oven to gas 4 / 180C / 350F and line a cupcake tin with paper cases

Mix the flour, baking soda and xanthan gum if using together in a bowl

Mix the lime / milk with the oil, vinegar and vanilla then add the sugar and stir until dissolved

Add the wet mix to the dry along with the lime zest and whisk until smooth

Divide the batter evenly between the paper liners

Bake for 25 minutes or until a toothpick in the middle tests clean then let cool completely on a wire rack

Frost the cooled cupcakes with a wide piping nozzle and sprinkle with lime zest to finish

To make the buttercream, beat the margarine until softened then add the powdered sugar, mixing until pale and fluffy before adding the lime juice and food colouring