Preheat the oven to 350 degrees F (175 degrees F). Grease only the bottom of a 9x5 inch loaf pan.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the buttermilk.
Combine the flour, 1/2 cup cocoa, baking powder, baking soda and salt; stir into the buttermilk mixture until moistened.
Mix in 1/3 cup pecans.
Pour into the prepared pan.
Bake for 55 to 65 minutes in the preheated oven, until a wooden pick inserted near the center comes out clean. Cool in the pan for 15 minutes before removing to a wire rack to cool completely.
Combine the margarine, 1/4 cup cocoa, and milk in a saucepan over medium heat. Cook, stirring, until melted and smooth.
Remove from heat, and mix in the vanilla and confectioners' sugar. Stir in the remaining 1/2 cup pecans.