Olive oil cakes have been around for thousands of years originating in the Mediterranean region where olive oil was cheaper and easier to come by than butter. There are numerous varieties as well, this one favoring an orange flavor.
Heat the oven to 350° F. Oil, butter, or spray a 9-inch cake pan that is at least 2 inches deep with cooking spray and line the bottom with parchment paper. (If your cake pan is less than 2 inches deep, divide between 2 pans and start checking for doneness at 30 minutes.)
In a bowl, whisk the flour, sugar, salt, baking soda and powder. In another bowl, whisk the olive oil, milk, eggs, orange zest and juice and Grand Marnier. Add the dry ingredients; whisk until just combined.
Pour the batter into the prepared pan and bake for 1 hour, until the top is golden and a cake tester comes out clean. Transfer the cake to a rack and let cool for 30 minutes.
Run a knife around the edge of the pan, invert the cake onto the rack and let cool completely, 2 hours. Decorate with sifted powder sugar, if desired.