Heat the oven to 325 F. Grease and flour a 10-inch one-piece tube cake pan.
Combine the butter and shortening in a mixing bowl and beat with an electric mixer until smooth and creamy. Add the sugar and beat for 3 minutes on medium-high speed, until light and fluffy.
Add the eggs, one at a time, beating well after each addition. Beat in the vanilla.
Sift the dry ingredients into a separate bowl and stir to blend.
Add about one-third of the dry ingredients to the mixing bowl and beat on low speed until combined. Add half of the milk and continue beating until combined. Repeat with another one-third of the dry ingredients and the remaining milk, then beat in the remaining dry ingredients just until blended. Avoid overbeating.
Spoon the batter into the prepared pan and spread to level the top.
Bake the cake for 60 to 75 minutes, or until a wooden skewer or toothpick comes out clean when inserted into the center.
Cool the cake in the pan for 30 minutes, then turn out onto a cooling rack to cool completely.