Beat butter and shortening at medium speed with an electric mixer until creamy. Gradually add sugars, beating until light and fluffy. Add eggs, 1 at a time, beating just until the yellow yolk disappears.
Sift together flour and baking powder; add to butter mixture alternately with milk, beginning and ending with flour mixture. Beat batter at low speed just until blended after each addition. Stir in vanilla and pecans. Pour batter into a greased and floured 12-cup tube pan. Place pan in a cold oven; set oven temperature at 300°.
Bake for 1 hour and 30 minutes to 1 hour and 45 minutes or until a long wooden pick inserted in center of cake comes out clean. Cool in pan on a wire rack 10 to 15 minutes. Remove from pan; cool completely on wire rack.
Don't preheat the oven for this scrumptious pound cake--it gets its start in a cold oven. To add a light dusting of powdered sugar, spoon a few tablespoons into a wire-mesh strainer and gently shake over the cake's surface.