Beat the butter, cocoa powder, confectioners', salt, vanilla and flour together on medium speed until a soft, smooth dough forms. Stir or knead in 1 cup of the mint chocolate chips.Shape the dough into a disk, wrap in plastic wrap and chill 30 minutes or overnight.Preheat the oven to 350°F. Line two baking sheets with parchment paper. Scoop rounded teaspoonfuls of the dough and shape into balls. Place 2 inches apart and cook 8 - 10 minutes.Cool cookies on wire racks. Once cooled melt the remaining 3/4 cup mint chocolate chips with the milk and whisk until smooth. Dip the cookies halfway and place on parchment. If the chocolate doesn't set up refrigerate for 20 minutes or until set.Store in a tightly covered container up to 1 week.