Preheat the oven to 180°C/350°F (160°C fan). Line a large baking tray with baking paper.
Add the sugar to the Thermomix bowl and mix for 20 sec/speed 10 to form icing sugar. Wait a few seconds for it to settle before carefully removing the lid.
Add the softened butter and vanilla extract and mix for 1 min/speed 4. Scrape down the bowl, then mix again for 1 min/speed 4 until pale and creamy.
Add the flour and custard powder and mix for 20 sec/speed 4. The mixture will be a little sticky but manageable. If not, place it in the fridge for 10-15 minutes.
Flour your hands, then take a small amount of mixture out of the Thermomix bowl and roll it into a ball (about walnut size). Place it on the baking tray. Repeat until all the mixture is finished. Allow a few centimetres between each ball.
Lightly flour the end of a wooden spoon, make an indentation at the top of each ball, and then fill it with a little jam. You could also use your forefinger or thumb to do this. Once finished, chill the Jam Drops in the fridge for 10 minutes before baking.
Bake in the oven for 10-12 minutesuntil slightly golden around the edges.
Cool on the baking tray for 10 minutes(the Jam Drops will be soft). Then, transfer to a baking rack to cool completely.
Notes
Storage: Store the Jam Drops in an airtight container in a cool place for up to a week.