Preheat your oven to 350°F (175°C) and grease your 9-inch round cake pan with a little olive oil or cooking spray.
In a large mixing bowl, whisk together the extra-virgin olive oil, eggs, whole milk, grated orange zest, freshly squeezed orange juice, and Grand Marnier (if using) until smooth.
In another bowl, sift together the all-purpose flour, granulated sugar, kosher salt, baking powder, and baking soda.
Gradually add the dry ingredients to the wet ingredients, stirring gently until just combined. Avoid overmixing.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean and the cake is golden brown.
Allow the cake to cool in the pan for about 10 minutes, then transfer to a cooling rack to cool completely.
Once cool, dust the top with confectioners' sugar. Slice and serve.