Preheat oven to 180 degrees celsius. Line two baking trays with baking paper.
Measure the plain flour and cornflour into the Thermomix bowl and sift for 30 seconds, Speed 3, to combine. Place the flour mixture into a separate bowl and set aside.
Place the room temperature butter, vanilla extract and icing sugar mixture in the Thermomix bowl. Beat for 30 seconds, Speed 4. Scrape down the sides of the bowl and mix for a further 30 seconds or until the mixture is pale, fluffy and smooth.
Add the flour mixture and combine on Speed 1, slowly increasing to Speed 4 until smooth and completely combined (if your mixture is too sticky, add a little more flour).
Roll mixture into two level teaspoonful sized balls and place 5cm apart on the prepared baking trays. Press the tops of the balls lightly with a fork.
Press the tops of the balls lightly with a fork. Bake for 15 minutes (do not overcook). Leave to cool on the trays for 10 minutes before moving to a wire rack to cool completely.
To make the filling, combine the butter, icing sugar, lemon rind and lemon juice for 30 seconds, Speed 4. Scrape down the sides of the bowl and mix for a further 30 seconds or until pale and smooth.
Join two biscuits together with 1 teaspoon of filling.