Heat oven to 180°C/350°F/Gas 4 and lightly grease and line 2 baking trays with greaseproof paper.
Mix the flour, bicarbonate of soda, cocoa, sugar and a pinch of salt in a large bowl.
Put the beaten egg, buttermilk and oil into a jug and mix well.
Pour into the dry ingredients with the boiling water.
Spoon 20 spoonfuls onto the 2 trays, well spaced.
Bake in the oven for 12 minutes or until firm to touch.
Remove from the trays by peeling off the greaseproof paper and allow to cool.
To make the frosting,beat the butter in a food mixer until very creamy.
Sieve in the icing sugar and vanilla extract and beat in one scoop at a time at No.1 setting on your food mixer. Give it a quick blast to mix mid way and then turn back down to No.1 setting.
Beat well until it sounds ploppy.
Sandwich two whoppie pies together using the buttercream frosting.
You can make the frosting the day before and leave to stand in kitchen. Beat well the next day before use.
We love the taste of coffee with chocolate so you can add a drop of coffee essence or a teaspoon of Irel coffee to half of the frosting mixture for a bit of variety