TO MAKE CAKE: Position a rack in the center of the oven and preheat to 350 degrees. Lightly butter and flour a 9x13 inch pan, tapping out the excess flour.
Whisk the flour, granulated sugar, baking soda and salt in a large bowl to combine. In a medium saucepan, bring the butter, margarine, water and cocoa to a boil over high heat, stirring to dissolve the butter and margarine. Pour into the flour mixture and whisk well. Add the sour cream then the eggs and vanilla, and beat well. Spread evenly in the pan.
Bake until the cake springs back when pressed in the center, about 35 minutes. Remove from the oven and place on a wire rack.
TO MAKE FROSTING: Bring the butter, milk and cocoa to a boil in a large saucepan over medium heat, stirring to dissolve the butter. Gradually stir in the confectioner's sugar, then the vanilla. Stir in the chopped pecans. Pour over the warm cake.
Cool the cake in the pan on the rack. Cut into pieces and serve directly from the pan.