Using an electric mixer with a paddle attachment, beat butter, cream cheese, and granulated sugar until light and fluffy, about 3 minutes.
Add vanilla and beat until mixed in.
Add the salt to the flour and then add the flour to the butter mixture in 2 additions, stopping to scrape down the sides of bowl. Beat just until the flour is incorporated into the mixture.
Place dough on a piece of plastic wrap, shape into a disk about 6 inches in diameter, wrap fully, and refrigerate for 1 to 24 hours.
Place oven racks in upper-middle and lower-middle positions and heat oven to 350 degrees. Line 2 baking sheets with parchment paper.
Break 5 of the chocolate bars apart at the seams so you have a total of 20 pieces.
Roll dough on a lightly floured surface into a 20 by 8-inch rectangle. Cut dough into twenty 2 by 4-inch rectangles.
Place a piece of chocolate crosswise across each piece of dough. Chocolate should overhang the edges some. Fold dough around chocolate and place on prepared baking sheets seam side down.
Brush tops with egg and sprinkle with sugar.
Bake for 18 to 20 minutes, until golden brown, switching and rotating baking sheets halfway through. Let cookies cool 5 minutes and then remove to wire racks to cool completely.
Break remaining chocolate bar into pieces, place in a microwave-safe bowl, and microwave on 50% power for about 1 minute. Stir and drizzle on cookies.