Preheat the oven to 180C/160C fan/gas 4.
Fill the baking trays with bun cases.
Make sure the butter is soft and mix it with the sugar in a bowl.
Sift in the flour.
Break the eggs into a separate bowl (spoon out any bits of shell that fall in) and add them to the bowl with the vanilla. Mix everything together.
Divide between the cases using a spoon, scraping it off with a knife.
Cook in oven for 20 minutes.
When cooked, leave to cool in the tray while you make the icing.
Mix the butter and icing sugar to make the icing.
Spread the icing onto each bun and decorate with sprinkles or whatever you like.