Preheat oven to 400 degrees.
Combine butter, shortening, cocoa, and water in a small saucepan.
Bring to a boil, stirring frequently.
Remove from heat.
In a large bowl, combine flour and sugar.
Stir in slightly cooled chocolate mixture.
Stir eggs into buttermilk.
Add soda and vanilla to buttermilk.
Stir buttermilk mixture into batter.
Pour batter into a 9 x 13 inch greased pan.
Bake 25 minutes.

Caramel Frosting:
Cook butter and sugar until slightly thickened.
Cool slightly.
Stir in milk and powdered sugar until spreadable.
Add vanilla.
Spread over cake.