For cupcakes : Put the butter, sugar, egg, flour, cornstarch, baking powder, cocoa powder and water in a bowl and beat for 2 minutes.

Divide this batter into 10 double thickness small paper cake cases (you may also use two paper cases for each to achieve this thickness). My cupcake cases were a bit larger so I got 8 cupcakes

Arrange five filled cupcake cases in a circle on your MW turntable. Cook at 100% for about 2 minutes. The cooking time will vary with the microwaves so check after about 1 1/2 minutes. A toothpick should come out clean when inserted. Repeat with the remaining five filled cases. My turntable is on the larger side so I put in all 8 cases in one batch.

Allow to cool.

For the frosting : While the cupcakes are cooling, put the butter in a MW safe bowl. MW at 60% for 40 seconds to melt the butter. Add the cocoa powder, boiling water and mix. Add the icing sugar while stirring constantly to obtain a smooth paste.

Spread the frosting over the warm cupcakes and decorate as preferred. Let the frosting set. Remove the extra paper case (if using) and serve.