Using the whisk on a hand mixer, beat the butter, sugar, vanilla seeds, salt and egg in a bowl until the mixture becomes lighter in colour. Mix in the flour and lemon zest. Place the dough in a plastic bag, press down firmly, refrigerate for approx. 30 mins.
On a lightly floured surface, roll out the dough in batches to approx. 4 mm thick. Cut out circles (each approx. 6 cm in diameter), place on two baking trays lined with baking paper. Cut little shapes out of half of the biscuits, chill for approx. 30 mins.
Approx. 12 mins. in a (convection) oven preheated to 170°C. Remove from the oven, allow to cool a little, then leave to cool completely on a rack.
Spread the jam over the biscuit bases. Dust the biscuit tops with icing sugar and place on top of the jam.