Preheat the oven to 350°F (175°C).

In a bowl, whisk together flour, baking powder, and salt.

Cream butter and sugar together until light and fluffy.

Add egg and vanilla extract to the butter mixture and mix well.

Gradually add the dry ingredients to the wet mixture and mix until a dough forms.

Divide the dough into two equal parts. Mix cocoa powder into one half to create a chocolate dough.

Roll out both doughs into rectangles of equal size. Layer them and roll tightly into a log.

Chill the dough log in the refrigerator for at least 30 minutes.

Slice the chilled dough into 1/4-inch thick cookies and place them on a baking sheet lined with parchment paper.

Bake for 10-12 minutes or until the edges are lightly golden.