For the walnut hearts, first make the shortcrust pastry. To do this, cut the butter into small pieces. Put the flour on the work surface or in a large bowl and add the sugar, vanilla sugar and salt. Together with the egg yolks and butter, quickly knead into a dough.
Shape the shortbread into a ball, wrap in plastic wrap and let rest in the refrigerator for about 2 hours before proceeding.
Preheat the oven to 170 °C hot air. Line a baking tray with baking paper.
Sprinkle some flour on the work surface and roll out the dough evenly on it. Use a cookie cutter to make heart-shaped cookies. Poke a small hole in half of the hearts. Place the cookie hearts on the baking sheet.
Whisk the egg yolks. Mix granulated sugar, nuts and cinnamon. Using a pastry brush, brush the cookies with pierced hearts with egg yolk and sprinkle the nut mixture on top.
Bake the hearts for about 12 minutes and then let cool slightly. Using jam, glue each cookie base together with a walnut heart.