Beat butter and shortening at medium speed with an electric mixer until creamy. Gradually add 3 cups granulated sugar, beating at medium speed until light and fluffy. Add eggs, 1 at a time, beating after each addition just until the yellow disappears.
Stir together flour and salt; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat at low speed just until blended after each addition. Beat in vanilla just until blended. Pour batter into a greased and floured 12-cup Bundt pan.
Bake at 325° for 1 hour and 15 minutes to 1 hour and 20 minutes or until a long wooden pick inserted in center of cake comes out clean. Cool in pan on wire rack for 10 to 15 minutes. Remove from pan; cool completely on wire rack. Dust cake evenly with powdered sugar.