1. Preheat oven to 350°. Grease and flour a 9- by 13-inch baking pan; set aside.
2. In a large bowl, cream together granulated sugar and 3/4 cup butter. Add eggs, buttermilk, and 1 teaspoon vanilla and beat until well blended.
3. In a small bowl, whisk 1/4 cup cocoa in 1 cup boiling water. Add to sugar mixture and beat to blend. In a medium bowl, combine flour, salt, baking soda, and cinnamon. Add to sugar-cocoa mixture and beat on low speed to combine thoroughly.
4. Pour batter into prepared baking pan. Bake 25 to 30 minutes, until a toothpick inserted in the center comes out clean.
5. In a 2-quart pan over high heat, whisk together remaining 1/4 cup cocoa and the milk and bring to a boil. Remove from heat and add remaining 1/4 cup butter, cut into small pieces, and stir until melted. Whisk in powdered sugar and remaining 1 teaspoon vanilla. Mix until smooth. Stir in pecans (if using). Pour mixture over the still-warm cake. Serve warm or at room temperature.