Preheat oven to 325°F. Grease and flour a standard 10-15 cup bundt pan, or spray with baking spray. Set aside.
In a medium size mixing bowl, use a whisk to sift together flour, baking powder, baking soda and salt. Set aside.
Using an electric mixer, cream together butter, 2 1/2 cups sugar and vanilla for 2-3 minutes until fluffy.
Add eggs one at a time, beating well after each addition. Add zest, beating until combined.
Mix together 1/2 cup orange juice with milk. Lower the speed of the mixer and add alternately with dry ingredients, beginning and ending with flour. Stop and scrape the sides of the bowl periodically.
Spread batter evenly in pan. Place into oven and bake for 75-80 minutes or until a cake tester inserted into the center comes back clean. Check halfway through baking and lay foil on top to prevent over browning, if needed.
To make orange syrup: Meanwhile, in a small saucepan, mix together remaining 1/2 cup sugar with 1/2 cup orange juice. Simmer over medium heat just until sugar is dissolved. Set aside.
When cake is finished baking, use a long skewer to poke holes in the cake. Spoon glaze over cake allowing to sink into warm cake. Cool cake completely in pan.
To make glaze: In a small bowl mix together powdered sugar, orange juice and zest until fully combined. Turn cake onto a platter or cake stand. Drizzle over cooled cake.
Store cake in an airtight container chilled or at room temperature.
You may dust this cake with powdered sugar and omit the glaze, if preferred.