Preheat oven to 350
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.
Grease cookie sheets. Bake in a 350 degree oven for 8-10 minutes or until edges are set. Cool on cookie sheet for 1 minute. Sprinkle with powdered sugar.