Place the egg whites in the bowl of a bain marie, ensuring the water doesn't touch the bowl. Gently heat to 72°c, whilst whisking the eggs continuously and adding the sugar gradually.
Once up to temperature, remove from the heat and continue to whisk until smooth, firm meringue peaks form (about 10 minutes).
Gradually add the softened butter whilst continuing to whisk. Continue to whisk for a further 10 minutes or until the icing is smooth and firm.
Slowly add the pre-prepared coulis until your buttercream is a light pink but not too runny - you may not need all the coulis.
Ensuring your cupcakes are completely cool pipe/spread the icing on top and enjoy!