Preheat oven to 350 °F and butter and flour a 9-inch round cake pan.
In a large bowl, whisk eggs and sugar. Add olive oil, milk and orange zest and mix well. In a separate bowl, stir flour, baking powder, baking soda and salt. Add to olive oil batter and stir just until smooth. Pour into prepared pan and bake for 40 to 50 minutes, until a tester inserted into the center of the cake comes out clean.
Let cake cool completely, then turn out onto a plate and dust with icing sugar.