Preheat the oven to 190 C (375 F). Cream the sugar, butter, salt and vanilla by mixing together. Next, mix in the flour, cocoa and ginger. Only add the milk if the dough is too dry. Roll the dough out very thin, preferably 5 mm or less. Cut out cookie shapes and place on baking sheet lined with parchment paper. Bake the cookies for 10 - 15 minutes.