Make the biscuit dough

Stir the flour and salt together then sift into a mixing bowl.

Rub in the butter until you have a mixture that looks like fine breadcrumbs.

Stir in the sugar.

Gradually stir in enough of the beaten egg to make a firm dough: you probably won't need all the egg.

Gently knead the dough into a ball, flatten slightly, then wrap in foil or cling film and chill in the fridge for at least 45 minutes.

Preheat your oven to 180°C /160°C Fan /Gas 4 /350° F.

Grease some baking trays or line them with baking paper. The mixture makes approximately 48 circles (i.e. 24 sandwiched biscuits) so you may need to cook in batches.

Cut out and chill the biscuits

When ready to bake, unwrap the dough and cut it in half. Re-wrap one half and return it to the fridge.

Put the other half on a lightly floured work surface and roll out thinly: approximately 2 - 3 mm (you can roll thicker for a chunkier biscuit but may need to increase the cooking time).

Using a 5cm fluted cutter, stamp out circles of dough. In the middle of HALF of them, cut out circles 1.5 - 2 cm in diameter. These will be the tops of your sandwiched biscuits, with jam peeping through the holes you made.

Tip: the end of a piping bag nozzle is approximately the right size to make the holes if you don't have a suitably sized cutter.

Put all the cut-out biscuits onto the prepared trays, leaving a little space between them. Gather the small circles you removed from the holes plus all the scraps and re-roll to make more biscuits, again cutting out the middles of half of them, until the dough is used up.

As you fill each tray, transfer to the fridge to chill while you make more: chilling means the biscuits are less likely to spread when baking.

Optional

Using the edge of a piping nozzle or similar, make decorative curved lines all the way around the holes (see images in blog post for how they should look).

Bake the biscuits

Put the tray(s) in the oven until cooked through and lightly golden (approx. 9 - 12 min). Watch them carefully as they can quickly burn and if your oven cooks unevenly turn the trays around halfway through.

When done, leave the biscuits on the trays for a minute or two, then carefully transfer to a wire rack and leave until completely cold.

Make the buttercream

For the buttercream, beat the butter then beat in the sifted icing sugar and the vanilla extract until soft and light.

Put in the fridge to chill a little.

Fill and sandwich the biscuits

Generously spread or pipe buttercream on the flat side of each base biscuit (i.e. those biscuits without a hole).

Put a top biscuit on each one, press down lightly, then spoon or pipe a small blob of jam into the hole in the middle.

Sprinkle with a little caster sugar.

Tip: If you prefer that the jam doesn't stand proud of the hole, then leave a space in the centre of the buttercream layer, fill it with a small amount of jam, then place the top biscuit on.

Stored in an airtight container the biscuits should stay crisp for 3 days. They will taste good for up to 5 days although the biscuits will be softer.

Unbaked biscuits can also be frozen. Allow longer to bake if cooking from frozen.