MethodLine a baking tray with baking paper and set aside. Heat the oven to 160°C. Beat butter and icing sugar until light and smooth. Add egg yolks, vanilla and lemon zest. Fold in the flour, cinnamon and salt. Roll into balls and place dough balls on prepared baking tray. Make imprints into the center of each ball. Spoon a small amount of jam into the center of each dough ball. Bake until golden brown, about 12- 15 minutes. Allow to cool and store in an airtight container.