Preheat oven to 325 degrees F (165 degrees C). Grease and flour an 8x4 inch loaf pan.
In a saucepan over medium heat, melt the butter, stir in the chocolate, remove from heat and stir until the chocolate is completely melted. Add the sugar and mix well. Combine the flour, baking powder and baking soda, stir into the chocolate mixture. Blend in the sour cream. Beat in the eggs one at a time, then stir in the vanilla. Finally, fold in the ground hazelnuts. Spread the batter evenly into the prepared pan.
Bake for 45 to 50 minutes in the preheated oven, or until a toothpick inserted into the cake comes out clean. Allow cake to cool for 10 minutes in the pan before inverting onto a wire rack to cool completely.
