For the walnut chocolate cookies, preheat the oven to 190°C degrees and line 2 baking sheets with baking paper.
Beat the butter with the brown and white sugar until fluffy. Beat the egg and stir it into the sugar-butter mixture. Mix the baking soda with 1/4 tsp water first and stir into the batter.
Mix the flour, cornstarch and salt with the walnuts and chocolate and slowly fold this mixture into the batter. Spoon the batter onto the baking sheet (leaving enough space between cookies) and bake the walnut chocolate cookies in the oven for 6 to 8 minutes until golden brown.