You will need:
2/3 cup butter, softened
1 cup sugar
1-1/2 teaspoons vanilla extract
1-1/2 cups all-purpose flour
1/2 cup cocoa
1/2 teaspoon baking soda
1/4 teaspoon salt
1/3 cup buttermilk or sour milk*
Heat oven to 350 degrees F. Lightly grease cookie sheet.
In large bowl, beat butter and sugar until well blended. Add egg and vanilla; beat until light and fluffy. Stir together flour, cocoa, baking soda and salt; add alternately with buttermilk to butter mixture.
Using ice cream scoop or 1/4 cup measuring cup, drop dough about 2 inches apart onto prepared cookie sheet. 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.