Using an electric mixer (stand or handheld), cream together butter, sugar, vanilla and salt until smooth, about 3 min. Scrape down sides of bowl, add egg and mix well. Scrape down sides of bowl again, slowly add flour and cocoa. On a sheet of plastic wrap, shape dough into a 1-in. (2.5 cm) thick rectangle, wrap and refrigerate until chilled, about 20 min.
Preheat oven to 350°F (180°C). Dust a piece of parchment paper with flour, place dough in centre, dust top of dough with flour and cover with another sheet of parchment paper. Roll dough to about a 1/4-in. (5 mm) thickness, dusting with more flour if necessary. Remove top sheet of parchment (use it to line your baking sheet), cut cookies into 1 x 2-in. (2.5 x 5 cm) bars and place on prepared baking sheet, spaced about 1 in. (2.5 cm) apart. Place baking sheet in refrigerator and chill, 10 min. Bake on middle rack of oven until firm, about 12 min. Transfer cookies to a cooling rack and let cool completely.
To make icing, melt chocolate in small microwaveable bowl on HIGH for 1 min., then whisk until smooth. Let cool to room temperature. In another bowl, using an electric mixer (stand or hand-held), beat together icing sugar, butter, milk and extracts until smooth and creamy, about 2 min. Add chocolate and mix until combined. Increase speed to medium and beat until fluffy, about 1 to 2 min. Spread or pipe 2 tsp (10 mL) on each cookie and let set before serving.
Use a different extract, like orange or almond, to create a whole new flavour.