Preheat the oven to 325°F and line a 9x9-inch pan with parchment paper.
Beat the egg whites in a large bowl, on medium speed.
Slowly add the sugar, then increase to high speed and whip until thick, glossy peaks form.
Reduce the mixer to low/stir and slowly add the orange juice, oil, egg yolks, orange zest, vanilla extract, and salt, mixing just until combined.
Combine the flour and baking powder in a bowl and stir a whisk through it to aerate and remove lumps.
Slowly blend the flour mixture into the batter, working it only until the flour is just incorporated into the batter.
Transfer to the prepared pan, spreading the batter into an even layer.
Bake the cake for 40–45 minutes. Start checking it around the 35-minute mark; the cake is done when a toothpick comes out clean and the top springs back lightly when touched.
Notes
If it starts to brown faster than you’d like, you can loosely cover it with foil for the last part of baking.Allow the cake to cool before serving. Dust with confectioners’ sugar and garnish with orange slices and/or fresh mint.