Steps

preheat the oven to 200°C.
beat the butter and sugar together until pale and fluffy, an electric whisk might be easier.
pour in the beaten egg, vanilla extract and buttermilk slowly and continue to whisk hard.
sieve in the dry ingredients and fold into the mixture gently until fully combined.
stir through the marshmallows and chocolate chips then scoop individual teaspoons full of the mixture onto a baking parchment lined tray, leaving plenty of space between each one for them to spread.
bake in the oven for 8 minutes then remove and leave to cool completely.
smear the flat side of half of the whoopies with a thin layer of nutella then stick another whoppie on top, flat side towards the nutella.
dust with some icing sugar to decorate and get your friends and family to tuck in.