Preheat oven at 180 C.
Line a big baking tray with tin foil or parchment.
In a large bowl, cream the butter and sugar-salt mix with a spoon for 2 minutes, adding in the vanilla extract, until well combined.
Sift the flours and baking powder together on a sheet of paper or mix it with a fork in a bowl and add it to the creamed mix.
Mix well until it forms a dough, adding 1-2 tsp of cold water or milk to bind, if required.
Divide roughly in half. Keep one half aside. To the other half, add the cocoa powder, a few drops of cold milk and knead gently to make a smooth but stiff dough (or you cant slice them).
Now you'll have two rounds of dough, one white and one brown.
Divide each into two logs, to get two white logs and two brown logs.
Join one white log and one choco log and roll into a log on a clean smooth surface. It's not necessary, that the white and brown parts are exactly half and half, don't stress out over this-just make sure each log as some white and some brown.
At this stage you could refrigerate for 15 mins or so and then slice into 1.5 cm thick slices or go ahead without chilling.
Place each slice on the baking tray leaving some space between each cookie.
Bake at 180C for 10-12 minutes, until the white parts start turning golden. At this point they will still be very soft, so don't overbake. They will crisp up as they cool.
For chewy cookies, remove them a little earlier.