Prepare the chopped chocolate. Chop the chocolate and put it in a mesh sieve to sift out the dust, if desired. Set the chopped chocolate aside.

Cream the butter and sugars. In a mixing bowl with an electric mixer, beat the butter until creamy. Add the brown sugar and confectioners' sugar and beat until light and fluffy.

Make the cookie dough. Beat in the egg yolks, vanilla, and salt. Reserve an egg white and refrigerate it. This will be used to coat the logs before baking. Refrigerate or freeze the remaining egg white to use another day, or discard it. With the mixer on low, add the flour and beat until combined. Fold in the chopped chocolate.

Shape logs and chill. Divide the cookie dough into 2 portions. Place 1 portion on a sheet of plastic wrap and roll it into an 8 to 9-inch log. Wrap it well with the plastic wrap and place it in an empty paper towel roll, if desired. Repeat with the remaining portion and refrigerate the logs for 3 hours or overnight.

Prep the oven and baking sheets. When it's time to bake, heat the oven to 350 F and line baking sheets with parchment paper.

Coat the logs with egg wash and sugar. Whisk the reserved egg white and brush over the log. Coat the log with sanding or sparkling sugar.

Slice and bake. Cut the logs into 1/2-inch slices and arrange them on the prepared baking sheet about 1 1/2 inches apart. Bake the cookies for 12 to 15 minutes, until the bottoms are lightly browned. Repeat with the second log of dough.