Preheat oven to 175 degrees celsius.
Place cupcake cases into the holes of two 12-hole muffin trays.
Place the butter, caster sugar, salt and vanilla extract into a bowl.
Beat together with a stand-mixer or hand-held beaters until pale and fluffy.
Add the eggs to a separate bowl and beat together lightly.
Slowly add the egg mixture to the butter mixture while continuing to beat.
When all of the egg mixture has been completely combined with the butter mixture, sift in the self raising flour and stir with a spoon until just combined (don't over combine).
Place the cupcake mixture into the cupcake cases - filling them to 2/3 full.
Bake in the oven for 12-15 minutes or until golden and when they spring back when touched in the centre (if you're making mini cupcakes, reduce the cooking time slightly).
Leave cupcakes in the baking trays for 5 minutes before moving to a wire rack to cool completely.
Cut a small hole in the middle of each cooled cupcake (not quite to the bottom). Discard the majority of the removed cake, keeping just a small slice for the lid.
Fill each hole with sprinkles, mini chocolates, Smarties, M&Ms etc.
Place the reserved slices of cupcake back onto the top as a lid (press down lightly).
To make the icing, beat the butter in a bowl until pale and fluffy (approximately 5 minutes).
Slowly add the icing sugar mixture, milk and vanilla extract and continue to beat until smooth and creamy.
Spread the frosting over the cupcakes and decorate with extra sprinkles.