Preheat the oven to 350 ℉ and line and grease an 8 or 9 inch springform cake pan.

In a large bowl, beat the sugar and egg yolks until light and foamy.

Add the orange zest, orange juice, oil, and almond extract, and whisk until well incorporated.

Mix in the almond flour and baking powder until combined; the mix should be smooth without any lumps.

In a separate clean bowl, beat the egg whites until firm peaks form.

Carefully fold the egg whites into the cake batter.

Pour the batter into the prepared pan, spreading it evenly.

Sprinkle the sliced almonds over the top.

Bake for about 40 minutes, until golden or until a toothpick inserted in the middle comes out almost clean with just a few crumbs.

Let cool in the pan for 10 minutes before removing from the pan.

If using a regular cake pan, invert the wire rack over the cake pan, hold it tightly, and quickly turn it over to release the cake. Place a serving dish over the cake and invert again to serve.

Sieve powdered sugar on top to decorate and serve.