80gpowdered icing sugar (confectioner's sugar 2/3 cup)plus extra for dusting
250gplain flour (all purpose flour, 2 cups)
Cream the butter and icing sugar together with an electric mixer until light and fluffy.
Add the egg yolk and vanilla and mix to combine well.
Mix in the flour until it makes a soft dough. It will be soft, but shouldn’t be too sticky to manage. If it is, mix in another tablespoon of flour.
Chill the dough, wrapped in clingfilm, in the fridge for at least 30 minutes.
Heat the oven to 180C/350F. Line a baking tray with parchment paper.
Roll half of the dough out (keep the other half in the fridge) on a lightly floured surface to about 1 cm thick.
Cut out rounds with an 7cm or 8cm cookie cutter. For half of the rounds, cut another, smaller shape in the middle to form a window. Re-roll any trimmings and make an equal amount of plain and windowed biscuits.
Place the biscuits on the baking tray and bake for 10 minutes. Allow the biscuits to cool on the tray.
Sandwich a small dollop of jam between the plain and windowed biscuits and then dust with sieved powdered icing sugar.
The dough will be very sticky before chilling. Just scrape it together as best you can and wrap.
The dough is very soft, so lightly roll it, shifting it gently after each roll so it doesn’t adhere to the countertop too much.
Use a flat spatula to gently lift the cutouts onto the baking tray.
Don’t overbake the cookies; they should be very pale and only very slightly golden.
Allow to cool completely before adding the jam of it will melt.