Preheat oven to 350°F. Line a baking sheet with parchment paper and set aside.
Cream the butter and sugar until light and fluffy, then beat in the egg. Divide the mixture evenly between two bowls.
Into the vanilla swirl bowl, add vanilla extract, flour and baking powder and mix to a soft dough. Into the chocolate swirl bowl, add the cocoa powder, flour, baking powder and vanilla extract and mix to a soft dough.
Dust two sheets of parchment paper with powdered sugar. Roll each ball of dough out into a rectangle of the same size.
Slide one layer on top of the other and roll into a tight log, starting with the long end. The dough is fragile so be careful (or refrigerate slightly before rolling out).
Wrap the log in parchment paper and refrigerate for at least 15 minutes.
Cut log into 1/4" slices. Place on prepared baking sheets and bake for 15 minutes, until the bottoms are golden. Transfer to a wire rack to cool completely.