Cream together the butter and sugar until light and fluffy. Add the egg and vanilla, mixing until combined.
Stir in the flour and pinch of salt until you have a soft, smooth dough. If it feels sticky, add a tablespoon more flour.
Chill the Dough
Flatten the dough into a disk, wrap it, and chill for 30 minutes. This helps keep the biscuits sharp-edged and prevents spreading in the oven.
Roll and Cut
Preheat your oven to 180°C (160°C fan).
Roll the dough out to about 5mm thick on a lightly floured surface.
Cut out circles using a cookie cutter.
For half the biscuits, cut a little heart or circle from the centre—this will be the classic “window.”
Bake
Place on a lined baking tray and bake for 8–10 minutes, until just turning golden at the edges.
Let them cool completely; they’ll firm up as they cool.
Prepare the Jam
Warm the jam gently on the stove or in the microwave until loosened. If you’d like a bit of tang (and to cut the sweetness), stir in a teaspoon of lemon juice.
Assemble
Spread a teaspoon of jam onto the base biscuits.
Top with the cut-out biscuits, pressing gently so the jam peeks through the window.