Mix the flour, soda, salt and cocoa powder together.
Finely chop chocolate and add to mixture.
Lightly whisk the eggs, yolk and vanilla extract and then pour into mixture, stirring to combine.
Add whole hazelnuts to this mixture, form into a log shape and chill slightly before baking. (Sometimes it is easier to shape the log by wrapping dough in plastic wrap).
Form into logs; brush with egg wash, sprinkle lightly with sugar.
Bake 30 minutes at 325-350 degrees.
Remove from oven and let cool for 10 minutes.
Slice into serving portions while still warm, return to a low oven, about 275-300 degrees to dry out.
Store at room temperature in an airtight container.