Turn on the oven to preheat it at 175C.
Mix the flour with the cocoa powder and the salt in a bowl and leave aside.
Put the butter and the chocolate (break the chocolate in small pieces) in a bowl and microwave it for 2 minutes (this will melt the chocolate and butter fast and easy). Whisk together until a smooth composition is obtained.
Beat the eggs with the vanilla essence, the sugar, then with the greek yogurt (I used a manual mixer for this process).
Then add the cholcolate-butter composition and mix thoroughly.
Add the flour-cocoa mix and incorporate gently with a spatula. Don't over do it :)
Chop the hazelnuts then add them to the prepared composition, mix gently again.
Prepare a medium baking tin with baking paper inside (leave the baking paper high on the edges - this will help you take out the brownies when ready).
Pour the brownies composition and evenly distribute it across the tin bottom.
Bake the brownies for 30 minutes.
Take out the tin and by pulling the edges of the baking paper, take out the brownies 'cake' on a cooling rack.
Slice into 15 pieces and leave to cool completely before serving.
Enjoy!