Preheat your oven to 180C (160C fan). Line a large baking tray with non-stick greaseproof paper.
Into a large bowl, weigh your flour and your butter. Rub them together between your fingers and thumbs until it turns into a breadcrumb kind of consistency. You can use a food processor for this if you like.
Add your icing sugar and continue to rub everything together. Using your hands or your machine, keep working your mix until it comes together into a dough. Once it’s formed, wrap in cling film and chill for at least 5 minutes and up to 48 hours. The longer the better.
Roll your dough out using plenty of flour to stop it sticking. You can make it as thin or as thick as you like. Cut out with a cookie cutter or upturned glass - you should get at least 12 rounds out of it.
Bake on your baking tray for 10-12 minutes. Your biscuits should just be blushing brown on the edges, and no more. Leave to cool – at this stage they’ll freeze well and keep for many days
To assemble, spread jam liberally on the underside of one round and stick on another one. Mix your icing sugar with enough water so that it is just gloopy – it’s very easy to go too far, but you can just add a little more sugar. Spread this on top of your biscuit and lay a gummy-sweet on whilst it’s still wet. These biscuits will be best about 24 hours after assembly, will freeze brilliantly and keep, in a sealed container, for many days. However, none of that matters because they’ll be gone within minutes, I’m sure.
Replace 50g of plain flour with 50g of corn flour for a much shorter, crumblier biscuit. Corn flour doesn’t have any gluten in it, and gluten is what tends to hold biscuits, breads and cakes together. Too much will make them stodgy!