In a medium bowl whisk together the flour, baking powder, and salt. Set aside.
3 cups all-purpose flour, 1 teaspoon baking powder, ¼ teaspoon kosher salt
In a large bowl, using a hand mixer, cream the sugar and butter together. Add the eggs and vanilla and mix well.
1½ cups granulated sugar, 1 cup unsalted butter, 2 large eggs, 1 teaspoon pure vanilla extract
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.
¼ cup unsweetened cocoa powder
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 baking sheet with parchment paper. Preheat oven to 400°F.
Cut the dough rolls into ¼-inch slices. Place the cookie slices 1 inch apart on the prepared baking sheet.
Bake 8-10 minutes, until the cookies are just set.
Immediately transfer to a wire rack to cool.