Preheat oven to 205 ºC (400 ºF) with rack positioned in the lower third of the oven.
Line 20cm x 20cm (8″x 8″) square metal baking pan with parchment paper.
In a double boiler over simmering water, melt together chocolate and butter. Stir until smooth and unified.
With a wooden spoon, stir in sugar, salt and vanilla.
Add one egg at the time to the mixture and incorporate by constant stirring.
Gradually add flour to the mix and stir until the mixture is homogeneous and without any lumps.
Add roasted hazelnuts and mix until evenly distributed throughout the batter.
Pour the batter into the lined pan and bake for about 30 minutes.
Put the pan in a cold bath and let it cool off for about 30 minutes. Be careful not to splatter water on top of the baked cake.
Cut baked cake into 16 squares and serve. Store at room temperature in a sealed container or covered with a plastic wrap to prevent drying out.
Hazelnuts Roasting
Spread hazelnuts over a parchment paper lined baking pan and roast at 175ºC (350ºF ) for 10-15 minutes or until fragrant and their skin is cracked.
Wrap the roasted hazelnuts in a kitchen towel, set them in a bowl and let them cool off for about 10 minutes.
Close and twist the towel creating a bundle with hazelnuts. Rub the bundle in circular motion against your palm or other flat surface. Through rubbing action against one another, the hazelnuts skin will come off in small flakes.
Transfer skinned hazelnuts to a large Ziploc bag. Crush hazelnuts to desired particle size by using a rolling pin.