Cream butter and sugar together until light and fluffy.
Add the egg and vanilla extract, mixing until combined.
In a separate bowl, whisk together flour, baking powder, cocoa powder, and salt.
Gradually add the dry mixture to the wet ingredients, mixing until the dough just comes together.
Divide the dough into two equal parts; add cocoa powder to one half.
Roll each portion of dough between parchment paper to form logs.
Chill the dough logs in the refrigerator for at least 2 hours.
Preheat the oven and line baking sheets with parchment paper.
Slice the chilled dough into even pieces and place on the prepared baking sheets.
Bake until set but still soft in the center, then cool on wire racks.
