In a large mixing bowl, beat the soft butter with an electric mixer for 1 minute or until smooth. Add the powdered sugar, brown sugar, vanilla and salt. Blend on medium until combined. Scrape down the sides and bottom of the bowl with a rubber spatula, and blend again.
Add the flour in four additions and mix on low speed until incorporated. The dough will seem a little dry, but keep mixing as it comes together.
Add the chopped chocolate to the cookie dough and mix together using a wooden spoon, or you can fold it in using your hands.
Place the dough on a large piece of parchment paper. Form the dough into a log 12-inches long and about 1 1/2-inches thick. Wrap the dough tightly in the parchment paper and refrigerate for at least 4 hours or overnight.
To bake preheat oven to 350 degrees F.
Line two baking sheets with parchment paper, set aside. Remove the dough from the refrigerator while preparing the egg wash. Whisk together the egg and water. Lightly brush the entire log with the egg wash. Pour the coarse sugar down the middle of a piece of parchment or wax paper. Roll the dough log in the sugar to coat, pressing down firmly to adhere. Using a large, thin, sharp knife, slice the log into 24 (1/2-inch) rounds. Place the cookies on the prepared cookies sheets, about 1-inch apart. They don’t spread much so no worries. Sprinkle the tops of each cookie with sea salt, if using. Bake for 16-20 minutes or until the they are starting to brown on the bottom and the cookies are set. Cool on a wire rack and store in an airtight container.