Preheat the oven to 350ºF. Spray or brush the inside of a 9-inch round baking pan with oil.
In a medium mixing bowl, combine the sugar and eggs. Beat on medium-high speed until the mixture is foamy and well combined. While mixing on medium-low speed, gradually add the milk, orange juice, and oil, mixing until they are all incorporated.
Whisk the almond flour, potato starch, baking powder, salt, and orange zest into the wet ingredients and mix well.
Transfer the batter to the prepared pan.
Bake for about 45 minutes, until a toothpick inserted into the center comes out clean.
Notes
You can substitute cornstarch or tapioca starch for the potato starch (just be careful with substitutions if making for Passover.)