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In a mixing bowl, combine flour, ground hazelnuts, sugar, vanilla, salt, butter, and yolks. Quickly knead into a smooth dough. Shape into a disc, wrap in plastic wrap, and chill for about 1 hour.
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Preheat oven to 325°F (160°C fan).
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Roll dough to about ⅛ inch (3–4 mm) thick on a lightly floured surface. Cut out cookies using a round cutter (about 2 inches wide). For half of them, cut out a smaller circle in the center for the “tops.”
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Bake for 10–12 minutes until lightly golden. Cool on a rack.
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Melt white chocolate with coconut oil. Drizzle or dip the top cookies and decorate with chopped hazelnuts while still soft.
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Spread hazelnut cream (loosened with liqueur if needed) on the bottoms and sandwich with decorated tops.