Preheat oven to 180 degrees C. Place the rack in the center of the oven. Butter a 20cm square pan, and line the bottom and sides of the pan with baking paper.
Melt the chocolate and butter in a large stainless steel or a glass bowl placed over a saucepan of simmering water. Remove from heat and stir in the cocoa powder and sugar. Whisk in the vanilla extract and eggs, one at a time, beating well after each addition. Finally stir in the flour, salt and hazelnuts.
Pour into the prepared pan and bake for about 25 minutes or until a toothpick inserted in the centre comes out with a little batter and a few moist clumps clinging to it. Do not over bake. Remove from oven and let cool on a wire rack. Serve at room temp or chilled.