With a whisk (electric is best unless you're a machine) beat together the butter and icing sugar until its smooth and combined.
Once combined, add in the vanilla extract, plain flour and egg yolks and whisk again until there is a dough!
Wrap up the dough and refrigerate for at least one hour.
Once the dough is chilled, preheat the oven to 180C/160C Fan and line two-four large baking trays with parchment paper.
With half of the dough, roll out the dough to roughly the thickness of 1-2 £1 coins and cut out 15 circles/hearts/shapes (Preferably a 6-8cm cutter) and then using a smaller little cutter (As you can see in the photos) cut out another little shape in the middle so that you'll see the jam once they're assembled.
Split them between the two trays and bake in the oven for 10 minutes!
Using the other half of the dough, and the little bits you cut out, roll the dough out to the same thickness and cut out 15 of the same shapes, but don't cut the middles out
Transfer to the other sheets and refrigerate for 10 minutes. Once chilled, bake the second batch in the oven for 10 minutes or until they're turning pale golden!
(If you have any more dough leftover then roll them out again like before and split between whole biscuits and patterned biscuits, and refrigerate them for at least 10 minutes before you bake them, handling the dough so much will warm it up and you want it to be cold when baking)
Leave the biscuits to cool on the baking tray
Once cooled, on the solid biscuits, spoon a larger teaspoon of jam into the centre and help it spread out, but not right to the edge.
Press the other biscuit on so that they are aligned - you should effectively have a jam biscuit sandwich (Like in the photos!)
Dust the biscuits with some icing sugar and enjoy!
Notes
You can use whatever flavour Jam you like, Strawberry is my personal favourite though and is what you can see pictured.
I made my centres into little hearts for Valentines, but you can make flower shapes, circles, squares, whatever you fancy!
These will last in an airtight container for up to one week but would also be a great topped for my Jammie Dodger Cupcakes!