Preheat oven to 350°F. If pie crust is frozen, place on a baking sheet and allow to thaw 15 minutes. In a large mixing bowl combine flour, sugar, cocoa, eggs, butter and vanilla. Stir well to combine. Fold in nuts and stir. Pour batter into shell. Place in oven (on baking sheet) and bake for 25 minutes or until toothpick inserted into center comes out clean. Top with whipped cream and cherries, if desired.