Preheat the oven to 350 degrees F. Spray a 9x13 baking pan with nonstick baking spray. I like to use the kind specifically for baking that has flour in it.
In a small bowl, whisk together the flour, baking powder, salt and soda. In a measuring cup, whisk together the milk, lemon juice and orange juice.
In the bowl of your electric mixer, beat together the butter and the sugar on medium speed until fluffy, for 5 full minutes. Scrape down the sides as needed. Beat in the orange zest until combined. Beat in the eggs 1 at a time until combined. Beat in the vanilla extract.
With the mixer on low speed, mix in half of the flour mixture. Mix in the milk mixture. Mix in the remaining flour mixture until just combined.
Pour the batter in the greased baking pan. Spread the top to even it out. Bake for 35 to 40 minutes or until a tester inserted in the center comes back clean.
Let the cake cool completely before frosting. I usually remove mine after about 30 minutes in the pan - I just invert it and frost the flat side. You can leave it in the pan and frost too!