Cut the butter into cubes and leave it out on the kitchen counter until softened. Zest and juice the lemon.
Sift the flour and baking powder in a bowl. Add sugar and butter and swiftly mix with your hands.
Add egg yolk, lemon zest and juice and mix until just incorporated and the dough forms a ball. Do not overmix or the dough will be dense. Wrap in cling film and place in the fridge for 1 hour.
Preheat the oven to 200 C / Gas 6. Roll out the dough 2 to 3mm thick and cut out circles of 7 to 8cm in diametre. Place a spoonful of plum jam in the centre of each circle and fold the dough over on 3 sides to form a triangle, leaving the centre open.
Place the pastries on a baking tray lined with baking parchment. Bake in the preheated oven for 10 to 15 minutes.