Preheat the oven: Set the oven to 325°F (165°C). Grease and flour a 10-inch bundt pan thoroughly to ensure easy release.
Cream the base: Beat butter, shortening, and sugar together until pale, fluffy, and creamy — about 5 minutes on medium speed.
Add the eggs: Beat in the eggs one at a time, ensuring each is fully incorporated before adding the next.
Mix the dry ingredients: In a separate bowl, whisk together the flour, baking powder, and salt.
Prepare the wet mix: In another bowl, combine the milk, orange juice, zest, and vanilla.
Combine everything: Alternate adding the flour mixture and milk mixture to the creamed butter, beginning and ending with flour. Mix on low speed until smooth. If desired, swirl in orange food coloring for a marbled interior.
Bake: Pour the batter into the prepared pan, smoothing the top. Bake for 1 hour 20 minutes to 1 hour 30 minutes, or until a toothpick inserted comes out clean.
Cool: Let the cake rest in the pan for 15 minutes, then invert onto a wire rack. Once cool, freeze for 1 hour — this helps the glaze adhere perfectly later.
