Preheat the oven to 180 C / Gas 4. Line cupcake tray with 12 paper cases.
Beat butter and sugar until light and fluffy. Then whisk in eggs one by one, then the vanilla. Fold in flour. Add any of the optional solid ingredients at thins point, such as chocolate chips or dried fruit. Pour mixture into the cases.
Bake for 20 minutes, or until a knife can come in and out of a cupcake cleanly. Cool on rack for 5 to 10 minutes.
Slice cupcake in half horizontally and put a dollop of custard in the middle. Here you can also add a little bit of jam or peanut butter, if desired. Put cupcake back together and enjoy!