Combine the ground walnuts with the butter, sugar, vanilla bean seeds, egg, salt, flour and cornstarch. Knead to form a smooth dough. Wrap in plastic and refrigerate for 30 minutes.
Preheat oven to 180°C (approximately 350°F). Line a baking sheet with parchment paper.
Roll the dough out to about 4-5 mm (1/4 inch) thin and cut using a star-shaped cookie cutter. Transfer onto the prepared baking sheet and gently press a walnut half onto the center of each cookie. Bake for 10-15 minutes, until golden brown around the edges.
Pour the vanilla sugar into a shallow dish. Allow the cookies to cool slightly before tossing in the vanilla sugar to coat.