In a small saucepan, combine the butter, oil and chocolate. Cook and stir over low heat until chocolate is melted. Remove from the heat. Add the water; pour into a mixing bowl. Cool to room temperature. Beat in the eggs, sugar and vanilla. Combine the flour, cinnamon, baking soda and salt; add to the chocolate mixture alternately with buttermilk. Pour into a greased 13x9-in. baking dish. Bake at 350° until a toothpick inserted near the center comes out clean, 30-35 minutes. Remove to a wire rack.
In a small saucepan, combine the butter, milk and chocolate. Cook and stir over low heat until the chocolate is melted. Remove from the heat. Stir in confectioners' sugar and vanilla until smooth; fold in pecans. Spread over warm cake. If desired, sprinkle with additional chopped pecans.
To substitute for each cup of buttermilk, use 1 Tbsp. white vinegar or lemon juice plus enough milk to measure 1 cup. Stir, then let stand 5 min. Or, use 1 cup plain yogurt or 1-3/4 teaspoon cream of tartar plus 1 cup milk.