Step 1Heat the oven to 325 degrees.
Step 2Beat the butter and sugar until they are light and fluffy, then add the eggs 1 at a time and egg yolk, beating well with each addition. Next beat in the vanilla and sour cream.
Step 3Combine 1 cup of the flour with the salt and baking soda and add it to the batter. Add the remaining flour 1 cup at a time until the dough is stiff, and then add the hazelnuts and chocolate. You may have to incorporate the last of the flour, nuts and chocolate by hand.
Step 4Divide the dough into 4 pieces and roll each piece into a log about 1 1/2 inches across. Set the logs on baking sheets and bake until they are golden brown, about 40 minutes.
Step 5Remove and slice each log diagonally about 1/2-inch thick.
Step 6Place the pieces cut side down on the pans and bake them until browned, an additional 30 to 40 minutes.