1. Preheat oven to 350°F (180°C). Line a 12x16-inch (30x40 cm) rimmed baking sheet with parchment paper.

2. In a large bowl, beat egg whites until soft peaks form. Gradually add granulated sugar, then confectioners' sugar, continuing to beat until stiff, glossy peaks form.

3. Gently fold in ground hazelnuts and flour in two additions, mixing just until uniform.

4. Spread batter evenly onto the prepared baking sheet, smoothing the top with a spatula.

5. Bake 20 minutes, or until the meringue spring back lightly when touched. Cool completely on a wire rack.

6. Once cool, carefully loosen edges and peel off parchment.

7. In a chilled bowl, whip heavy cream to stiff peaks. Set aside 1/2 cup for garnish.

8. Flip the meringue onto a clean sheet of parchment so the bottom side faces up.

9. Spread the whipped cream evenly over the surface, leaving a 1-inch border on one long edge.

10. Drain mandarin segments and arrange in a single layer over the cream, reserving a few pieces for garnish.

11. Starting from the long edge opposite the border, roll the meringue tightly into a log. Transfer seam-side down to a serving platter.

12. Chop chocolate and melt using a double boiler or microwave in 10-second bursts, stirring until smooth.

13. Let chocolate cool slightly, then drizzle over the top of the cake.

14. Pipe or spoon reserved whipped cream along the center. Garnish with reserved mandarin segments.

15. Chill 30 minutes before slicing. Serve cold.