Sift together flour, baking soda and salt in a medium bowl.
In small bowl, mix together buttermilk and vanilla.
Beat butter and white sugar together in a large mixing bowl with an electric mixer for about 3 minutes or until evenly distributed and light in color. Add egg to butter and sugar mixture and beat until blended. Gradually mix in flour mixture one cup at a time, adding buttermilk mixture between each cup of flour, mix until smooth. Scrape down the sides of the bowl as necessary.
Drop leveled 1/4-cups of batter onto a greased baking sheet. Bake on middle rack for 14-16 minutes, or until the tops are golden brown.
Allow cookies to cool completely before icing.
In a mixing bowl, beat butter until creamy. Add sugar and cocoa, mixing well. Add milk in 2 portions, stirring each time. Add vanilla extract.
Prepare a pastry bag with a multi-opening/ grass tip, fill with the frosting and close tightly with a rubber band. Hold the tip vertically over the cookie, squeeze and pull up quickly. Repeat to cover the whole cookie.