Preheat your oven to 350F and line a small square pan with baking paper.
Mix the melted butter with sugar then stir in the cocoa powder, salt, flour and baking soda and give it a good mix. The mixture will be pretty dry at this point.
Mix the eggs with the vanilla extract then add them over the cocoa batter and mix well.
Fold in the chocolate chips and walnuts then pour the batter in your prepared pan.
Bake in the oven for 20 minutes, maximum 25 minutes.
Allow to cool in the pan then slice into small squares and serve.