2 sticks (1 cup) unsalted butter
1/2 cup Dutch-process unsweetened cocoa powder
3/4 cup water
2 cups granulated sugar
2 large eggs
1/2 cup well-shaken buttermilk
2 tablespoons vanilla
2 cups all-purpose flour
1 teaspoon baking soda
1/2 teaspoon cinnamon
1/4 teaspoon salt
2 cups chopped pecans (71/2 ounces)
1/2 stick (1/4 cup) unsalted butter
1/2 cup half-and-half
1/2 cup confectioners sugar
5 ounces fine-quality bittersweet chocolate (not unsweetened), finely chopped
1/4 teaspoon salt
Special equipment: a 9-inch tube pan or 12-cup bundt pan