Preheat oven to 325 degrees F. Grease and flour a Bundt pan.
With a mixer, cream together softened butter, shortening, sugar, eggs and vanilla until smooth.
Combine flour, baking powder, salt and cocoa powder; alternately add dry ingredients with milk to the creamed mixture.
Pour into prepared Bundt pan. Bake for 1 hour and 20 minutes or until toothpick comes out clean.
Let cool in pan for 15 minutes; loosen edges with knife and remove from pan.
To prepare glaze, mix softened butter, powdered sugar, milk and vanilla. Pour over cooled or slightly cooled cake.