Add butter, sugar, vanilla extract and salt into a medium bowl. Beat with a hand held electric beater, for about 2 minutes, until smooth and creamy.
Beat in flour, until just combined. Divide dough in half and remove one half to a plate (this is the vanilla dough). Add cocoa powder to the remaining half in the bowl and continue beating with the beater (begin low and increase to medium), until it is evenly combined to make chocolate dough.
Roll - Shape vanilla dough into a rectangular disc and roll between two pieces of parchment paper to ¼-inch thickness. Similarly, roll chocolate dough separately to ¼-inch thickness.
Gently transfer vanilla dough on top of the chocolate dough. Beginning on the long side, roll the dough up tightly jelly-roll style. Using both hands, roll the log gently to make it a bit longer (this helps to make both doughs adhere well and make medium sized cookies), refer video.
Chill - Wrap log with the parchment paper or plastic wrap and refrigerate overnight (or up to 3 days).
Bake - Pre heat oven to 350°F and line a large baking sheet with parchment paper.
Remove dough from refrigerator, unwrap and cut into ¼-inch slices using a sharp knife. Place on the prepared baking pan, spacing about 1 and ½ inches apart (cookies will expand slightly but not too much).
Bake cookies for 20 to 25 minutes (rotate pan half way through baking), until cookies are golden in color. Place pan on a wire rack to cool completely.