Pre-heat your oven to 180°c (or 160°c for a fan assisted oven) and line a couple of baking trays with greaseproof paper.
Cream your butter & sugar until light & fluffy.
Add the egg and mix until combined. Sift in the flour and mix to form a dough - you may need to use your hands to help the dough come together.
Roll the dough on a floured surface to around 1cm thick. Using a round cutter, cut your biscuits & transfer to your baking tray. The number of biscuits you will get will vary depending on the size of cutter you choose to use.
Bake for around 10 minutes until the edges of the biscuit are just golden. Again, depending on the size of your biscuits, you may need to bake them a little bit longer.
When still warm but cool enough to touch, add the jam to the under side of half your biscuits and pop the other half on top of these to sandwich together.
When completely cool, make your icing by mixing icing sugar & gradually adding the boiling water until you're happy with the consistency - "gloopy" is a great way to describe it! Gently dip the top of your biscuits into the icing to cover and top with your decoration of choice!