1. Heat oven to 350°F. Lightly grease cookie sheet or line with parchment paper.
2. Beat butter and sugar in large bowl until well blended. Add egg and vanilla; beat until fluffy. Stir together flour, cocoa, baking soda and salt; add alternately with buttermilk to butter mixture. Using 1/4 cup (#20) ice cream scoop or 1/4 cup measuring cup, drop dough about 2 inches apart onto prepared cookie sheet.
3. Bake 13 to 15 minutes or until cookie springs back when touched lightly in center. While cookies are on cookie sheet, sprinkle lightly with additional sugar. Cool slightly; remove from cookie sheet to wire rack. Cool completely. Makes about 12 cookies.
* To sour milk: Use 1 teaspoon white vinegar plus milk to equal 1/3 cup.
NOTE: This is an old-fashioned cakey sugar cookie. Lactose free milk may also be used.