Weigh the all-purpose flour and sift it into a large bowl. Add the baking powder and the salt and whisk the ingredients together.

Chop the chocolate into small uniform pieces and place it in a small microwave safe bowl. Microwave the chocolate in two 30 second bursts stirring the chocolate after each 30 seconds. Then microwave in 15 second bursts stirring after each 15 second interval. When there a couple of pieces left stir the chocolate until it is all melted. Set aside to cool.

Place the butter and sugar in the bowl of a stand mixer. Mix on medium low speed until the ingredients are combined and the butter and sugar look creamy, about 1 minute Scrape the bottom of the bowl. Add the egg and vanilla extract and beat on medium speed until the egg is fully incorporated.

Add all the dry ingredients to the batter and mix on low speed until the ingredients are thoroughly combined and the dough starts to form a ball. The dough will weigh 530 grams.

Weigh out 280 grams of the dough and put into a small bowl. This dough is your vanilla dough.

Tear off four pieces of wax paper, each about 14 inches long. On one piece, place one of the doughs and shape it into a square. Cover it with another piece of wax paper. With a rolling pin, roll over the paper to roll the dough out to 1/8-inch thickness. It should be about 9 by 9-1/2-inch square. Trim any pieces of dough that exceed the 9-inches to square the dough and add the excess dough to the corners so it the dough is square. Set the first piece of dough aside and repeat these steps with the second piece of dough.

Remove the dough from the refrigerator. Lay the vanilla dough a on work surface. Peel one side of the wax paper off the dough then put it back on. Turn the dough over and then peel off the top piece of wax paper only. Repeat these steps with the chocolate dough – peel off one side of the wax paper, then put it back on. Turn it over and peel off the other piece of wax paper. Use the wax paper to help lay the chocolate dough on top of the vanilla dough. Line the edges up as much as possible.

Use the wax paper under the vanilla dough to help roll the doughs into a cylinder. Trim the ends so the cylinder has straight ends. Wrap the dough in the wax paper and twist the ends to close the wax paper.

Chill the dough for 60-90 minutes.

Right before the dough is removed from the refrigerator, preheat the oven to 350°F. Line one or two half baking sheets with parchment paper.

Unwrap the dough and place it on a cutting board. Cut the dough into 1/4-inch slices. Place the slices 1 inch apart on the baking sheet. Bake for 10-12 minutes until the bottom edges of the cookies barely turn a light brown. While the first set of cookies is baking return the other sliced cookies to the refrigerator.

Remove the cookies from the oven, let them sit on the cookie sheet for about 2 minutes then use a spatula to move the cookies to a wire cooling rack.

If using only one baking sheet, let it cool completely before baking the next batch of cookies. If using two baking sheets put cookies on the second baking sheet and repeat the baking instructions.

Store the cookies in an airtight container for up to 10 days.