Preheat oven to 350 degrees and grease a 9X13-inch pan.
Combine 1 cup butter, 1/3 cup cocoa, and 1 cup water in a small saucepan and heat over low heat, stirring constantly, until butter is fully melted. You can cut the butter into pieces to speed up the process. Remove from heat.
Using an electric mixer, beat buttermilk, eggs, baking soda, and vanilla extract until smooth.
Gradually add melted butter mixture, beating until mixed.
In a medium bowl, combine sugar, flour, and salt. Add to buttermilk mixture and beat until blended. Pour batter into prepared pan. Bake for 30 to 35 minutes, or until set in the middle.
To make frosting, combine butter, cocoa, and buttermilk in a medium saucepan. Heat over medium heat, stirring constantly, until mixture is smooth.
Remove from heat and stir in confectioners' sugar, vanilla, and pecans. Pour over cake while cake is still warm. Let frosting cool and set some before slicing.