In a mixing bowl, cream together the butter and sugar until light and fluffy.
Add the egg and vanilla extract, and mix until combined.
In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add to the butter mixture, mixing until just combined.
Divide the dough in half. Add cocoa powder to one half and mix until fully incorporated.
Roll out both dough halves into rectangles of equal size. Place the chocolate dough on top of the plain dough and roll tightly into a log.
Wrap the dough log in plastic wrap and refrigerate for at least 1 hour.
Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.
Slice the dough log into ¼-inch thick slices and place on the prepared baking sheet.
Bake for 10-12 minutes, or until the edges are lightly golden. Cool on a wire rack.