Preheat the oven to 360 degrees F. Butter a 9 by 13-inch baking pan.
Add flour, sugar, baking powder, orange zest, orange juice, eggs, milk, and oil. Beat with a hand mixer until thoroughly mixed together, about 3 to 4 minutes.
Add the batter to the buttered pan and bake for about 25 minutes. Test the cake with a toothpick inserted into the center. If it comes out clean, the cake is done.
Let cool for about 30 minutes on the counter, then flip the cake out of the baking pan. Slice and serve sprinkled with powdered sugar.