Making the muffins: Heat the oven to 170°C. Take a muffin tray and line each cup with a paper muffin case. Place the butter and chocolate in a small pan. Pour the milk in a small bowl and sprinkle with the coffee granules. Stir well until the coffee is dissolved then pour into the pan. Warm on low heat until the butter and chocolate are completely melted. Take off the heat immediately and let the mixture cool down.
Pour the flour into a large bowl. Remove 1 tablespoon of the flour and add 1 tablespoon of cocoa powder instead. Add the bicarbonate of soda and the sugars. Stir well. Rub the mix between your fingers to eliminate the lumps caused by the muscovado sugar. In a medium bowl, whisk the eggs then add the Greek yogurt. Tip into the flour and sugar bowl then pour the melted chocolate. Stir a few times, using a large metal spoon, until it becomes light and airy. Be careful not to over stir or the mixture will harden.
Pour the dough into the muffin cases. Put in the oven for 20 minutes. The muffins should be well-risen with a firm top.
Making the coffee icing: In a bowl, cream the butter and icing sugar. Pour the milk in a small bowl and dissolve the coffee in it. Add to the butter and sugar mixture and beat until you get a smooth icing. Take a large piping bag with a large star nozzle and fill it with the icing. Squeeze to create swirls on top of the cupcakes.
Decorating: Sprinkle the chocolate vermicelli on the icing and top with Easter eggs.