Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a mixing bowl, whisk together the flour, baking powder, and salt. Set aside.
In another bowl, cream the butter and sugar until light and fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Divide the dough in half. Mix the cocoa powder into one half of the dough until well combined.
Roll out both doughs into rectangles of equal size. Place the chocolate dough on top of the plain dough and roll them together into a log. Wrap the log in plastic wrap and refrigerate for at least 30 minutes.
Slice the log into ¼-inch thick cookies and place them on the prepared baking sheet.
Bake for 10-12 minutes or until the edges are lightly golden. Let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.