Preheat oven to 350°F and grease a 9"x13" cake pan.
Combine butter, cocoa and water in a small sauce pan. Use low heat, stirring continuously until butter is completely melted.
In a large bowl beat buttermilk, eggs, baking soda and vanilla extract with an electric mixer until smooth.
Gradually pour melted butter cocoa mixture into the bowl with the buttermilk mixture, beating until combined.
In a medium bowl combine sugar, flour and salt. Gradually add to buttermilk mixture and beat until smooth.
Pour cake batter into a greased 9"x13" pan. Bake for 30 to 35 minutes or until the center is set.
Remove from the oven and place on a wire rack to cool.
Notes
Chocolate Buttermilk Icing Recipe
1 cup unsalted butter
1/4 cup unsweetened cocoa or unsweetened dark cocoa
1/3 cup buttermilk
16 ounces powdered sugar
1 teaspoon pure vanilla extract
1/2 cup chopped pecans
In a medium sauce pan combine butter, cocoa and buttermilk. Use medium heat stirring constantly until butter is melted and mixture is smooth.Remove from heat and stir in powdered sugar, vanilla extract and chopped pecans.Pour over cake while it is still warm. Let cool completely before slicing.