1. Make cake: Preheat oven to 350°F. Coat a 9-inch round cake pan (at least 2 inches deep) with olive-oil cooking spray. Line bottom with parchment; mist with spray.
2. In a bowl, whisk flour, baking powder and salt. In a separate bowl, combine sugar, zest and eggs. Using an electric mixer on high speed, beat mixture until pale and thickened, about 5 minutes. Reduce speed to low and beat in orange juice. Gradually add olive oil, beating constantly. Stop mixer and use a rubber spatula to fold in dry ingredients. Pour batter into prepared pan and bake until a toothpick inserted into center comes out clean, 35 to 40 minutes. Let cool in pan on a wire rack for 15 minutes. Run a small knife around edge of cake, then turn onto rack to let cool completely.
3. Make glaze: Whisk confectioners' sugar, milk and vanilla in a small bowl until smooth. Set cake and rack over a foil-lined rimmed baking sheet. Spread glaze on cooled cake, allowing it to drip over edges. Let glaze set for at least 20 minutes before serving.