Step 1

Combine the butter and sugar in the bowl of a stand mixer. Beat on medium speed for 2 to 3 minutes, or until light and fluffy. Add the egg, egg yolk, and vanilla, then mix on low until fully, scraping the sides of the bowl as needed.

Step 2

Whisk together the flour, baking powder, and salt in a medium bowl. Add to the butter mixture and mix on low speed until fully combined and there are no flour pockets.

Step 3

Divide the dough in half, reserving half in a separate bowl. Add the cocoa powder to the other half, mixing until no streaks remain in the dough.

Step 4

Place each half of the dough on a separate piece of parchment paper and press into a 1/2-inch thick square. Place another sheet of parchment paper over the dough and roll into a 9-by-9-inch square. Chill both squares for about 30 minutes or until the dough is firm but still malleable.

Step 5

Once the dough is chilled, remove the top piece of parchment paper from both squares and flip the chocolate dough on top of the vanilla, lining up the edges as much as possible. Remove the top parchment and trim the edges to create even sides. Carefully roll the dough into a log (like a jelly roll), using the parchment paper as a guide and peeling back the paper as it is rolled. Wrap the roll in plastic wrap, twisting the ends like a candy wrapper to press the dough together firmly and remove any air trapped inside.

Step 6

Refrigerate the dough for about two hours. If the log needs some reshaping, remove the dough from the refrigerator after 30 minutes of chilling and gently roll it on the counter to work it into a round cylinder. Return the dough to the fridge to finish chilling.

Step 7

Preheat the oven to 375º and line two sheet trays with parchment paper.

Step 8

Once the dough is firm, unwrap the log and roll it in your choice of walnuts or sprinkles. Trim the round edges, then carefully cut the log into 24 cookies, about 1/4 inch thick, using a sharp knife. Bake 1 1/2 inch apart on the prepared trays for about 8 to 10 minutes, until the tops look dry. Let cool for 2 minutes on the sheet tray, then transfer to a wire rack to cool completely.