Preheat the oven to 170°C (325°F/Gas 3). Have a 30 × 40 cm(12 × 16 inch) baking tray ready — you can line it if you like, to help you lift out the shortbread when it’s cooked, but it’s not absolutely necessary.
Put the butter and sugar in a good-sized bowl and work them together by hand or with a wooden spoon until combined. Add the flour and baking powder and work them in. Add the egg and vanilla and knead them in until it is all compact and smooth. Cover with plastic wrap and leave in the fridge for at least half an hour until the dough is firm enough to roll out.
Divide the dough in half. Roll out one half on a lightly floured surface so that it will fit into your tray. It should be 2–3 mm thick (about ⅛ inch). Fit it into your baking tray, making sure that it is a fairly even thickness all over. Spread the jam over the top, as if you were spreading it over a slice of toast. Roll out the other half of the dough and fit it as exactly as possible over the bottom one. If it is difficult to lift, roll it loosely over your rolling pin and carry it that way. It isn’t essential that all your edges are exact; you can break off a bit from here and patchwork it in there. It will taste the same.
Bake for about 15 minutes, or until the shortbread is golden in places. The edges will start to turn golden brown first, followed by the top. Remove from the oven and cool for 5 minutes in the tray. Lift out of the tray, using the baking paper.
Cut into shapes with a cookie cutter, or just into squares or diamonds. Or you can leave it in one piece and keep cutting chunks out of it as you go past. It will keep in a biscuit tin for 5 or 6 days.