Place the flour, bicarbonate of soda and salt in a small bowl and stir together. Place the butter in a large bowl, add the brown sugar and beat until the mixture is very light and fluffy. Add the egg, egg yolk and vanilla and beat until the mixture is smooth. Add the cocoa mixture and mix until smooth. Using a metal spoon, fold in the flour. The mixture will be pretty runny so don’t be alarmed. Pour this chocolate batter evenly into the liners (I pour the batter into a large jug – it’s much easier with runny mixtures) I pour 60ml of batter into each one (using a 1/4 cup measure)Bake for 18-22 minutes or until the cakes are risen and they spring back when lightly pressed. Transfer the cakes to a wire rack and leave to cool. While the cupcakes are cooling, make the buttercream.