Knead the ground walnuts, butter, sugar, vanilla, egg, salt, flour and cornstarch into a smooth dough. Wrap the dough in plastic and chill for about 30 minutes.
Roll out the dough until 3-4 mm (approximately 1/4 inch) thick. Use a cookie cutter to cut out about 90 circles. Place the cookies on a lined baking sheet. Bake for 10-12 minutes at 180°C (approximately 350°F). Remove from the oven and let cool.
Heat the jam and pass through a sieve. Spread the jam on half of the cookies and gently place the remaining cookies on top of the jam. Mix the powdered sugar and lemon juice into a thick icing. Pour the icing into a pastry bag and decorate the cookies. Top each cookie with a walnut half, let dry and serve.