In a small bowl stir together flour and baking soda; set aside. In a medium saucepan melt butter; remove from heat. Stir in granulated sugar, cocoa powder and brown sugar. Stir in buttermilk and vanilla. Stir in flour mixture just until combined. Cover and chill dough for 1 hour. (Dough will be stiff.)Advertisement
Preheat oven to 350 degrees F. Lightly coat cookie sheets with nonstick cooking spray or line with parchment paper. Drop chilled dough by rounded teaspoons onto cookie sheet.
Bake for 8 to 10 minutes or until edges are set. Cool on cookie sheet for 1 minute. Transfer to a wire rack and let cool. Sprinkle with powdered sugar.
Tip: To make 1/4 cup sour milk, place 3/4 teaspoon lemon juice or vinegar in a glass measuring cup. Add enough milk to make 1/4 cup total liquid; stir. Let mixture stand for 5 minutes before using.
1 other carbohydrate