Preheat your oven to 350°F (175°C) and prepare two baking sheets by lining them with parchment paper.

In a large mixing bowl, cream together 1 cup of softened unsalted butter and 1 ½ cups of granulated sugar for about 3-5 minutes until light and fluffy.

Beat in 1 large egg and 1 teaspoon of vanilla extract into the creamed mixture until fully combined.

In a separate bowl, whisk together 2 ½ cups of all-purpose flour, 1 teaspoon of baking powder, and ¼ teaspoon of salt.

Gradually add the flour mixture to the butter mixture, mixing on low speed until just combined.

Divide the dough evenly into two portions and place one in another bowl.

Add ⅓ cup of unsweetened cocoa powder to the second portion of dough and mix until fully integrated.

Roll out the plain dough into a rectangle approximately ¼ inch thick between two sheets of parchment paper.

Repeat with the chocolate dough to match the size of the plain dough rectangle.

Carefully layer the chocolate dough over the plain dough, aligning edges.

Roll up the layered dough from one long side using parchment to keep it tight.

Wrap the rolled log in plastic wrap and refrigerate for at least 30 minutes.

Slice the log into ¼ inch thick rounds once firm.

Arrange the cookie rounds on prepared baking sheets, spacing a couple of inches apart.

Sprinkle tops with colored sugar or crushed peppermint candies, if desired, before baking.

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

Let the cookies cool on the baking sheet a few minutes before transferring them to a wire rack to cool completely.