In a mixing bowl stir together the flour, baking powder, and salt; set aside.
In a separate mixing bowl, cream the butter, brown sugar and granulated sugar until smooth, about 2 minutes. Add the egg and vanilla and mix well.
Add the flour mixture to the butter mixture in 2-3 smaller additions, mixing well after each.
Divide the dough in half into two equal portions. Add the cooled melted chocolate to one of the portions, then mix well until the dough is a uniform light brown color.
Shape each of the doughs into square logs, about 2” by 2” by 10”. Wrap each tightly in plastic wrap and chill in the refrigerator for at least 4 hours or overnight.
Once the dough is chilled, use a sharp knife to trim the logs to be as straight and even as possible. Reserve the trimmed dough.
Slice each of the dough logs into 3 even slices. Lay the slices cut side down and slice each into 3 additional slices. You should have 9 strips of chocolate and 9 strips of vanilla.
Create a checkerboard pattern by alternating the strips of chocolate and vanilla dough and stacking to create a 3x3 checkerboard pattern.
Roll the reserved dough between two sheets of parchment paper until it will wrap around the cookies, then tightly wrap one checkerboard log with chocolate and the other with vanilla, using the parchment paper to help guide the rolled dough onto the logs
Tightly wrap each checkerboard dough log with plastic wrap, then return to the fridge to chill up for an additional 30 minutes-1 hour.
Preheat the oven to 350°F/175°C, line a baking sheet with parchment paper, and set aside.
Slice the cookies into ¼” slices, and place cut side down on the prepared baking sheet. Bake for 8-10 minutes, or until slightly puffed and lightly golden.
Notes
To make the logs even, use a kitchen scale to divide the dough in half before adding the chocolate.
When making the logs of dough, try to make each of them the same size and shape.
It's important to slice the dough evenly, otherwise, the checkerboard pattern won't line up properly. Use a long sharp knife, and a ruler if you need to.
Don't press too hard when combining the slices, or you'll distort the pattern. Only press enough to get the dough to touch.
This dough will get sticky if it is handled too much. Return it to the fridge for a bit if this happens.
The border made with the excess dough is optional, but I love the way it looks! Plus you won't waste anything doing it this way.
Store these cookies in an airtight container at room temperature for up to 5 days, or freeze for up to 3 months.