Preheat oven to 350°F/175°C.
Toast the nuts gently over medium heat in a skillet for 2-3 minutes
Melt the chocolate and butter in a microvave oven on medium effect, alt. use a pot on low heat on the stove and let it cool.
Whipp the sugar, vanilla, salt, and eggs into a fluffy mixture.
Chopp the hazelnuts coarsely.
Mix sourdough starter and nuts with melted chocolate and butter and add it into the bowl with sugar and egg. Fold everything together until almost combined
Pour everything into a baking tin, about 10x7-inch/25x18 cm, lined with parchment paper.
Bake for 25 minutes. Let the brownies cool for 30 minutes before slicing into squares.