Cream together the butter and sugar.
Add the egg and mix well.
Sift in cake flour, bread flour, baking soda, and salt, and mix until combined.
Gently fold in dark chocolate chips and walnuts.
Let the dough rest in the refrigerator for 30 minutes.
Shape the dough into balls and place them on a baking tray.
Bake in a preheated oven at 180°C (356°F) for 13 to 15 minutes.