In a small saucepan, cook butter, water and cocoa over low heat, stirring constantly, until butter melts and mixture is smooth. Remove from heat.
With an electric mixer, beat buttermilk, eggs, baking soda and vanilla until smooth. Add butter mixture and beat until blended.
In a separate bowl, combine sugar, flour and salt. Gradually add to buttermilk mixture, beating until blended.
Pour into a greased 15x10" jelly roll pan. Bake at 350 for 15-20 minutes until cake is set.
Meanwhile, In a saucepan, cook butter, cocoa and buttermilk over medium heat, stirring constantly until butter is melted and mixture is smooth.
Remove from heat and stir in powdered sugar, vanilla and pecans.
Spread Frosting over warm cake. Cut into squares, while warm.