Step 1

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.)

Step 2

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.

Step 3

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.