Preheat the oven to 350°F (180°C). Grease and flour a baking pan or cake tin.
Separate the eggs. In a clean bowl, whisk the egg whites with a pinch of salt until stiff peaks form.
In a separate bowl, cream together the egg yolks, granulated sugar, and vanilla sugar until pale and fluffy. Add the melted butter and orange juice, and mix well.
In another bowl, combine the flour, baking powder, rolled oats, and grated orange zest.
Gradually add the dry ingredients to the egg yolk mixture, then gently fold in the beaten egg whites until just combined. Be careful not to overmix, to keep the batter light and airy.
Pour the batter evenly into the prepared baking pan or cake tin.
Bake in the preheated oven for about 30-35 minutes, or until golden brown. Check for doneness by inserting a toothpick into the center; if it comes out clean, the cake is ready.
Remove from the oven and let cool slightly before dusting with powdered sugar and slicing to serve.
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