Preheat the oven to 180°C (350°F) (gas 4).
Beat the butter and sugar together using an electric hand mixer or food mixer. Once the mixture is smooth and creamy, gradually beat in the egg and then the vanilla extract.
Sift the remaining ingredients into the bowl and mix in until just combined.
Now tip out onto a work surface and gently knead by hand until a soft dough has formed.
Roll the dough out on a floured surface to 0.5 cm (¼ inch) thick. Then using the large cutter, cut rounds from the dough and place them onto the baking trays/cookie sheets.
Keeping all the whole rounds on one tray and the centre cut ones on the second tray, use the smaller cutter to remove the centres of half the rounds. (Don’t try to remove the centres from any whole rounds before placing on the trays/sheets as they will become misshapen when lifting them on.)
Keep re-rolling the leftover dough and cutting out the rounds, making sure there are an equal number of whole rounds and cut rounds. There should be enough dough to make about 15 finished biscuits.
Bake the centre-cut rounds for 8–10 minutes and the whole rounds for 12–14 minutes, or until the biscuits are just starting to brown. Transfer to a wire rack and leave to cool.
Spread a thick layer of strawberry jam over the whole rounds and then place the cut rounds on top.
Finish by dusting with icing/powdered sugar. The sugar will melt on the jam but will remain on the biscuit.
Store in an airtight container.