Preheat oven to 350ºF and prepare a 15-cup Bundt pan with butter and flour or spray.

Sift flour, baking powder, and salt into a medium bowl and set aside.

Using an electric mixer, cream together butter, shortening, and sugar. Add eggs one at a time, beating well after each addition.

Add the dry ingredients to the butter mixture in three parts, alternating with the milk, beginning and ending with the flour mixture. Mix at low speed until just blended. Add lemon juice and zest.

Pour the batter into the prepared pan and bake for about 55 minutes, until a tester inserted into the center comes out clean.

Cool the cake in the pan for 15 minutes, then turn it out onto a rack to cool completely.