Preheat oven to 180 degrees celsius (fan-forced).
Grease and line an 18cm x 28cm rectangular slice tray with baking paper - allowing the edges to overhang - and set aside until needed. (See notes)
Place the plain flour, caster sugar and the chilled butter into the Thermomix bowl and process for 10 seconds, Speed 5 or until the mixture resembles breadcrumbs.
Add 1 egg and the vanilla extract, and mix for 30 seconds, REVERSE, Speed 2 or until the mixture has formed into a dough.
Press the dough firmly into the base of the prepared pan using the back of a spoon.
Bake for 15 to 20 minutes in the preheated oven, or until it starts to turn golden.
Spread the jam evenly over the warm base. (See notes)