Rich, chocolatey and best reserved for special occasions!
This will easily cover a cake up to 30cm / 12inches, and is delicious on fairy cakes / cupcakes etc. too. You can also use to sandwich a cake together - it should do two layers if you don't spread it too thickly. The cake won't need to be refrigerated, although you can do if you wish, and the icing will become firm if you do.
It's very quick to whip up and easy to spread or pipe. Here it's been spread rather generously on a chocolate 'Magic Bean Cake' (recipe here).
85g dark chocolate (e.g. Green & Black's 70% cocoa)
120g icing sugar (golden/unrefined if possible, or mill your preferred sugar in Thermomix to a powder - reduce to 90g to for a richer topping for a magic bean cake!)
110g softened butter (reduce to 80g along with reduced icing sugar for magic bean cake. Keep the chocolate amount the same)
1 tsp vanilla extract
(See below for Thermomix method)
Melt chocolate (either in a microwave, in short bursts, or in a glass bowl over a pan of hot water) and set aside to cool down. Sift icing sugar, or pulse if using food processor to remove any lumps then cream with the softened butter, until smooth. Add the vanilla extract and melted chocolate, and pulse again (if using food processor, or stir in by hand) until incorporated evenly. Spread on your cake or cupcakes as desired once they are completely cool, then store in a cool place.
If milling your own icing sugar, do this first then set aside. Weigh the 85g of dark chocolate into the bowl, then blitz on Speed 8 / 10 seconds (MC on). Melt 30 seconds / 50C / Speed 2.5. Scrape down and add the butter, icing sugar and vanilla extract, and cream together for 15 seconds at Speed 4, scrape everything down, and mix for another 5 seconds, Speed 4. Spread on your cake or cupcakes as desired once they are completely cool, then store in a cool place.