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. About 1 dozen 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.