In a medium bowl whisk together the flour, baking powder and salt. Set aside.
In a large mixing bowl, cream together the sugar and the butter. Add the eggs and vanilla and mix well.
Add the flour mixture to the creamed butter mixture and mix until a soft dough is formed.
Divide the dough in half.
Add the cocoa to one half of the dough and mix until combined.
With a rolling pin, roll the chocolate dough into a rectangle about 9x14 inches, on a lightly floured board.
Repeat with the plain dough. Place the plain dough on top of the chocolate dough.
Gently roll the two doughs together to a 3/16 inch thickness. Use a sharp knife to trim the edges so that they are straight.
Beginning on the long side, roll the dough up tightly jelly-roll style.
Wrap in parchment paper or plastic wrap and put in the freezer for 1 hour.
Line a cookie sheet with parchment paper. Heat oven to 400F.
Cut the dough rolls into ¼ inch slices. Place the cookie slices 1 inch apart on the prepared cookie sheet.
Bake 8-10 minutes, until the cookies are just set.
Immediately transfer to a wire rack to cool.