Grease and flour a 15 x 10 x 1 inch pan.
In a large bowl combine flour, sugar, baking soda,and salt.
In a medium saucepan combine 1 cup butter, cocoa, and 1 cup of water. Bring to a boil, stirring constantly. Remove from heat. With electric mixer, mix the chocolate into the flour mixture. Add the eggs buttermilk and vanilla. Mix until lumps are gone. Pour into pan.
Bake at 350 for 25 minutes or until wooden toothpick comes out clean.
About 10 minutes before the cake is done, combine the 1/4 cup butter and cocoa into the saucepan. Add the 3 T buttermilk. Stir constantly until mixture is warmed. Remove from heat...add the rest of the ingredients. Pour over the warm cake.