In a medium bowl, sift together cocoa, flour, and salt.

In a stand mixer fitted with the paddle attachment, beat butter on medium until smooth and creamy. Gradually add sugar and beat, scraping down the bowl as needed, until creamed. It doesn't need to be light and fluffy, just well mixed. Mix in vanilla.

Reduce the mixer to low and add the flour-cocoa mixture in three additions, mixing just enough to incorporate between additions.

Fold in chopped chocolate.

Scrape dough onto a work surface and shape into two logs, each about 1 3/4 inch in diameter. The dough will be dry, so you may have problems shaping. Wrap in plastic and chill overnight.

Preheat oven to 350°F. Line two baking sheets with parchment or silicone baking mats.

In a small bowl, whisk together egg and water. Pour decorating sugar on waxed paper set in a baking sheet. Brush each log with egg wash and roll in sugar to coat.

Slice logs into 1/2-inch thick disks and place about an inch apart on the baking sheets.

Bake 20 to 23 minutes, rotating sheets front to back and top to bottom halfway through baking, or until the cookies are just firm to the touch. Let cool on baking sheets.