In a small separate bowl mix salt with the milk and vanilla extract to dissolve. then add to the creamed butter.
In another bowl, whisk together the cocoa powder and flour, then slowly combine this flour mixture in with the wet ingredients bowl. Once the dough is well mixed, toss in the chocolate chips.
Knead the dough lightly, enough to evenly distribute the chocolate chips. Split the cookie dough in half and roll the dough into two logs about 2 inches thick. Wrap each log shape in plastic wrap and refrigerate for 2 to 3 hours to firm up.
Preheat oven to 350 degrees and line baking sheets with parchment paper.
Cut each cookie dough log into slices about ⅜"-½" thick. Arrange on the prepared baking sheets. These shortbread cookies will not expand much at all, but leave a little space between them.
Bake for 11-12 minutes, then remove to wire rack to cool completely.