Line two oven trays with baking paper. In a large mixing bowl, beat the butter and sugars with an electric mixer until pale and creamy. Add the egg and vanilla extract and beat until combined. Sift in the baking soda, plain flour and cocoa powder. Add half the buttermilk and gently fold with a spatula, before adding the rest of the buttermilk. When the mixture is combined, it will be quite wet – similar to cake batter.

Cover the batter with plastic wrap and pop into the freezer for 30 minutes. This will help firm up the mixture so you can easily roll spoonfuls of the batter into balls using your hands – just like you would for cookies. They will be a bit sticky but rolling them in your hands will help achieve a nice smooth finish.

Preheat the oven to 360 F (180 C). Place the cookies onto the prepared trays, leaving space for them to spread out when cooking. Bake for approximately 8-10 minutes. Leave them to cool for a few minutes and then transfer the cookies onto a wire rack. While they are cooling, you can make the Nutella frosting.

To make the icing, beat the butter with an electric mixer until creamy. Add the icing sugar, one cup at a time, and beat with the electric mixer until smooth and creamy. Add the vanilla and beat again. Then add the Nutella and mix through, adding the milk as needed. The frosting should be fairly thick and easy to apply.

Pipe the icing onto one whoopie pie before placing another on top. Continue until all whoopie pies are complete.