Preheat: Heat the oven to 200C (180C fan / Gas 6 / 400F). Grease three baking sheets.
Cream: Put the butter and sugar into a large bowl and cream together until light and fluffy. Mix in the beaten egg.
Combine: Sift over the flour and salt, and fold in until combined. Stir in the chopped walnuts and chocolate chunks until evenly distributed.
Shape: Use a tablespoon to scoop out walnut-sized pieces of dough and roll them roughly into balls between your hands. Place on the baking sheets about 5cm apart and press down with a fork to flatten.
Bake: Bake for 8 to 10 minutes or until lightly golden around the edges and still soft in the middle.
Cool: Cool slightly on the baking sheets, then using a spatula, transfer the cookies to a wire rack and leave to cool completely. Store in an airtight container for up to 5 days.
