Sift or whisk together the flour, cocoa and baking soda.
In a mixing bowl, beat butter until creamy. Beat in both sugars, the salt (if using) and vanilla and beat for another minute or two. Reduce speed to low or by hand (which is how I do it), stir in the dry ingredients. Stir only until the dry ingredients are incorporated, then add the chocolate.
Turn the dough out onto a smooth work surface, divide in half. Shape each half into a cylinder that's 1 1/2 inches in diameter. Wrap in plastic and chill for at least one hour. You can also freeze.
Preheat oven to 325 degrees F. Line a baking sheet with parchment paper.
Slice logs into 1/2 inch thick slices. They'll probably break as you slice, so just pinch the dough back together and nudge into circles.
Place the slices on the parchment-lined sheets, leaving about 1 inch of spread space between each cookie. Bake one sheet at a time for about 12 minutes. The cookies will not look done, nor will they be firm, but that's just the way they should be.
Transfer the baking sheet to a cooling rack and let the cookies rest, on the baking sheet, until they are only just warm or until they reach room temperature. Repeat with the second sheet of cookies.
Keeping: The dough can be made ahead and either chilled or frozen. In fact, if you've frozen the dough, you needn't defrost it before baking - just slice the logs into cookies and bake 1 minute longer.