First, cream together the butter, sugar, and vanilla extract. Stir in the egg until well combined. Then stir in the flour and salt until a dough ball forms. Then, stir in the chopped chocolate.
You may need to use your (clean) hands to press the dough together here. Roll the dough into a long cylinder, cover in plastic wrap, and freeze for at least 2 hours or overnight. You can keep this frozen in your freezer for at least a couple months (probably longer).
When you are ready to make cookies, remove from the freezer and allow to sit out for a few minutes while your oven preheats. Then, slice into 1/4 inch or slightly thicker rounds and place on a baking sheet lined with a baking mat or parchment paper.
Bake at 325°F for 12-15 minutes in the upper 1/3 of the oven. Remove from the oven and place the cookies on a cooling rack.