Preheat oven to 350˚.
Place butter, water, and cocoa in microwave safe dish. Microwave 3 minutes, stirring once about halfway through.
Add sugar and flour and whisk until smooth.
Add eggs and buttermilk, salt and soda and mix well. Stir in vanilla.
Prepare pan (or pans) by spraying generously with baking spray.
Pour batter into pan or pans. (For muffin pans, I used about ¼ cup batter per cake.) Bake 25-30 minutes for 11" x15" pan or 15-25 minutes for cake pans or muffin pans. When the cake is done, it will bounce back when lightly touched. Another way to tell if it is done is to insert a toothpick into the middle of the cake and pull it out. If the cake is done, the toothpick will come out clean or with a few crumbs, not coated with batter.
Remove from oven and frost as directed below.
For the frosting, place butter and baking chocolate in microwaveable bowl. Microwave on high for 1 ½ minutes or until chocolate is melted. Stir until blended with a wire whisk.
Add milk and vanilla. Mix well. Slowly add powdered sugar and whisk until smooth. Frosting should be thick but still pourable when warm. If too thin add a bit more powdered sugar. If too thick add a little extra milk.