Preheat oven to 350 degrees and butter and flour two 8 or 9 in. cake pans.Whisk together flour, baking soda and salt.Over medium heat, combine chocolate, 1/2 cup sugar and 1/2 cup buttermilk for 5 minutes. Remove from heat and cool for 5 minutes.Beat together butter and remaining sugar. Add eggs and egg yolk. Beat in vanilla, then slowly add the flour and remaining buttermilk. Stir in the chocolate and blend well.Divide mixture evenly between two pans and bake for 30-40 minutes.