Pre-heat your oven to 180 degrees celsius (fan-forced) and line two baking trays with baking paper.
Place the butter and caster sugar into your Thermomix bowl and mix for 15 seconds, speed 3 to combine.
Scrape down the sides of your bowl and mix for a further 15 seconds, speed 3 or until pale and creamy.
Add the self raising flour, egg and vanilla extract and mix for 10 seconds, speed 4.
Select the kneading function and mix for 40 seconds.
Remove the bowl from your machine and use a table spoon to scoop up the mixture and roll into a ball. Place onto your baking trays. Repeat until all of the mixture has been used.
Lightly press down on top of each biscuit to flatten slightly. Place into the oven to bake for 10 - 12 minutes or until the edges of the biscuits start to turn golden.
Remove from the oven and let the biscuits cool on the oven trays for 10 minutes before transferring to a wire rack to cool completely.
Once completely cool, combine your icing ingredients in a bowl. Add one tablespoon of boiling water and then add an additional ½ tablespoon at a time until your icing is a spreadable consistency.
Use a butter knife to place the icing onto each biscuit and add the sprinkles.