• Preheat oven to 350° F and line a baking sheet with parchment paper.
• In a medium bowl, cream butter and sugar until creamy. Make sure your butter is at room temperature.
• Beat in egg yolk. Place egg in a bowl of warm water to bring to room temperature.
• In another bowl, mix flour, baking powder and salt.
• Whisk milk and vanilla in small cup.
• Alternate wet and dry ingredients until well blended.
• Divide dough in half. To equal out the quantities of vanilla and chocolate dough, remove two tablespoons of dough from one half and add to other half.

For the Chocolate Dough

• Mix your 3 tablespoons of cocoa powder and 1 tablespoon of oil in a small bowl. If you have the unsweetened chocolate square you can melt one square which is the equivalent. Since I don't bake often with unsweetened cocoa, I find it easier and more cost effective to keep cocoa powder such as Hershey's on hand as it has a longer shelf life then the squares. If you do buy the squares, I find they store longer in a drawer in the refrigerator.
• Blend the unsweetened chocolate with smaller half of dough. Once you add the unsweetened chocolate you will have equaled it out again.
• For pinwheels. Dust the wax/parchment paper and rolling pin with flour and roll each section into 1/4 inch think rectangles.
• Place chocolate layer over vanilla or visa-versa depending on which one you want as the outer layer of the pinwheel. Using your wax/parchment paper, tightly roll your dough into a cylinder shape.
• Cover and chill dough for 2 hours or overnight.
• Remove dough from refrigerator and slice into 1/4 inch slices. Save any end scrapes to press flat into a cookie.
• Tip: No matter which shape you are making, I like to rotate my dough every couple of cuts as the cutting action tends to pull the shaped dough downwards. This way your squares and circles will stay true.
• Line a baking sheet with parchment paper and evenly space each cookie. If your shape looks sloppy, just apply slight pressure on opposites sides. They will make 12-15 depending on the thickness of your roll. These cookies should not spread so you can use a half size baking sheet.
• Bake for 10-12 minutes. Remove these cookies before they get a golden brown. Let cool slightly before you remove them so they don't break. Store in a covered container on the counter.