Preheat the oven to 180C/160C Fan/Gas 4 and line 2 x 12-hole muffin tins with 18 paper cases.
Sift the flour, cocoa powder, baking powder and bicarbonate of soda into a large mixing bowl. Add the caster sugar and a pinch of salt.
Add the sunflower oil, eggs and milk and beat until smooth. Add the boiling water and mix again until silky smooth.
Scoop the batter into a large mixing jug and pour evenly between the paper cupcake cakes. Bake on the middle shelf of the preheated oven for 20–25 minutes, or until well-risen and a wooden skewer inserted into the middle of the cakes comes out clean.
Leave the cakes to cool in the tin for about 5 minutes, then transfer to a wire rack to cool completely.
Meanwhile, make the buttercream. Beat the butter, icing sugar and cocoa powder together using an electric hand whisk, until pale and fluffy. Pipe or spread the buttercream over the top of the cupcakes and scatter over the chocolate decorations. Store in an airtight container.
For the cupcakes
175g/6oz plain flour
40g/1½oz cocoa powder
1½ tsp baking powder
½ tsp bicarbonate of soda
250g/9oz caster sugar
100ml/3½fl oz sunflower oil
2 large free-range eggs
2 tbsp milk
125ml/4fl oz boiling water
For the buttercream
200g/7oz unsalted butter, softened
130g/4½oz icing sugar
15g/½oz cocoa powder
handful chocolate decorations