Put the flour, cocoa powder and butter in a bowl and rub in with the fingertips until the mixture resembles fine breadcrumbs. Add the sugar, egg yolks and vanilla extract and mix to a smooth dough. Wrap and chill for at least 30 minutes before using.
Roll out the dough on a lightly floured surface, then cut out rounds using an 8.5 cm (3¾ inch) round biscuit cutter. Use a 3 cm (1¼ inch) round cutter to cut out the centres to make rings. Place on greased baking sheets, spaced slightly apart, and re-roll the trimmings to make extras. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes or until beginning to darken around the edges. Cool on a wire rack.
Melt the white chocolate in a heatproof bowl over a pan of gently simmering water. Using a potato peeler, pare off curls from the plain dark chocolate.
Drizzle a little white chocolate over each cookie and scatter with some hazelnuts, rose petals and dark chocolate curls. Leave in a cool place to set for about 1 hour before using. They will keep, in an airtight container in a cool place, for a few days.