In the bowl of your stand mixer fitted with the paddle attachment, mix the butter and cream cheese together for 2 minutes, until smooth and creamy.
Add in the egg, the yolk, vanilla, baking powder and salt and mix for 1 minute until smooth, scraping the sides of the bowl as necessary.
Turn the mixer to low and add in the flour, mixing until just combined.
Divide the dough in half, removing half from the mixing bowl.
Add in the cocoa powder to the remaining dough in the mixing bowl and mix until incorporated.
Form each dough into a squared off log, 2 inches tall and 6 inches long. Make sure both are the same size.
Place dough on a baking sheet and cover with cling wrap. Place dough into the refrigerator to chill for at least 1 hour.
Using a sharp knife, slice the dough into thirds, lengthwise. Then turn the dough one turn and cut into thirds again. You will have 9 strips. Repeat the process with the other dough. You will have 18 strips total.
Lay 1 piece of vanilla dough, one piece of chocolate, and another piece of vanilla on a flat surface next to each other. Top the 3 strips with alternating colors. Repeat this once more, so you have a square log. Press the dough strips together tightly, keeping the square shape. Repeat this with the remaining dough.
Refrigerate again for at least 30 minutes.
Preheat the oven to 350 degrees. Line a baking sheet with Reynolds Kitchens® Parchment Paper with Smart Grid®.
Remove the dough from the refrigerator and slice into 1/4-inch pieces.
Place the pieces on the prepared baking sheet.
Bake for 9–10 minutes, or until set. Transfer to a wire rack to cool completely
Store airtight for up to 5 days. Freeze airtight for up to 30 days.