(click on the text to highlight each stage):

1. Preheat the oven to 180°C (fan 160°C)/350°F/Gas Mark 4. Line two 12-cup muffin tins with cupcake papers.
2. Sift the cocoa and mix with the vanilla in a small bowl. Set aside. Beat the butter and sugar together in a large bowl, using an electric hand mixer set on medium-high speed, or a free-standing mixer.
3. Once the butter and sugar mixture is pale, light and fluffy, add the egg yolks, one at a time, and beat until everything is combined. Add the cocoa mixture and beat well for another minute to combine.
4. Stir the buttermilk and salt together and add it to the butter and sugar mixture, a third at a time, alternating with the flour. Mix the bicarbonate of soda with the vinegar and blend into the batter. Then, with your mixer on high, beat everything together on a high speed for about 5 minutes, until you have a smooth, glossy batter.
5. Fill each cupcake case to three-quarters full. Bake for 18–20 minutes or until a skewer inserted in the cupcake centre comes out clean.
6. Cool in the tins on a wire rack for 10 minutes. Remove the cakes and cool completely before frosting.
7. For the frosting, whisk the milk, flour and salt in a small saucepan over a medium heat until the mixture thickens and begins to bubble, about 1–2 minutes. Transfer to a small bowl and allow to cool.
8. Melt dark or white chocolate. Set aside to cool.
9. Beat the butter, sugar and cocoa (if using) together until fluffy. Add to the cooled chocolate followed by the milk mixture and vanilla extract (if using). Beat together until smooth, then spoon into a piping bag with a small plain or star tube. Pipe immediately on to the cupcakes.
10. Tip - It is really important that an electric hand-held or freestanding mixer is used to blend the cupcake batter thoroughly, so that a loose, glossy mixture is achieved.