Ingredients ( makes approx 18)
Sift the flour and baking powder into a bowl, add the sugar and mix to combine. Roughly chop the butter and rub it into the dry ingredients with your fingertips (or whizz in a food processor) until the mixture resembles breadcrumbs. Add the vanilla seedsor extract. With the mixer running slowly, add the whisked egg a little at a time until a dough starts to form.
Tip onto a baking traylined with baking paper and knead gently for about a minute to bring the dough together. Shape into a rough rectangle, then cover with cling film and chill for at least 1 hour.
Turn the chilled dough onto a sheet of cling film and roll out to thickness of about 5 mm, trying to keep it roughly rectangular. Spread generously with jam, then gently roll up from the long side, using the cling film to help you.
Cut the log in half, wrap each piece in cling film and chill again for at least another hour.
Preheat the oven to 180ºC/Mark 4 and line two baking trays with non-stick baking paper. Unwrap the chilled logs and cut into slices no thicker than 1 cm.
Place them on the prepared trays, spacing them at least 2cm apart, and bake for 10–15 minutes, until starting to become golden.
Remove from the oven and sprinkle with caster sugar. Leave to cool on their trays.