Mix the flour, lemon zest and salt in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Beat the egg white, icing sugar and vanilla paste well, add and quickly combine to form a soft dough. Shape into a roll approx. 4 cm in diameter, cover and chill for approx. 1 hr. Cut the dough into slices approx. 5 mm thick, place on two baking trays lined with baking paper. Brush the shortbreads with egg yolk.
Mix the oats with the remaining ingredients. Spoon 1 tbsp onto each shortbread, press down lightly.
Bake each tray for approx. 15 mins. in the centre of an oven preheated to 200°C. Remove from the oven, allow to cool a little, then leave to cool completely on a rack.