This is a rich buttercream icing made with the addition of custard. It can be embellished by adding various flavoured liquors, aromatic essences and food colouring. This is my basic time-tested recipe for decorating cakes and cupcakes. Makes enough buttercream for one 3 to 4 layer cake or 24 cupcakes.
It is important that custard is completely cooled, otherwise it will melt the butter. The custard can be made ahead and chilled in the fridge till needed.
This recipe contains quite a bit of butter, for ease of decorating with a pastry bag and to help the piped icing to hold its shape. If you don't intend to create elaborate decorations and just want to simply ice your cake, you can decrease the butter to 200g.
For chocolate version add 1 Tablespoon cocoa-powder or Nutella. You can omit vanilla, but I really like to add 1 Tablespoon of rum or cognac to this buttercream. Other liquors can create interesting variations too: coconut (Malibu), orange (Grand Marnier), lemon (Limoncello) - depending on the cake you have in mind.