Preheat oven to 350°F (177°C). Line the bottom of an 8 or 9 inch springform pan with parchment paper. Spray with non-stick cooking spray and set aside.
With your fingertips, work the orange zest with the sugar, until the sugar has moistened and has turned an orange colour and is fragrant.
In a medium bowl, sift the flour and baking powder together. Set aside.
In a large bowl, using a stand mixer or a hand-held mixer, mix the egg yolks, orange scented sugar, vanilla and orange blossom water on medium-high speed for 2 minutes, or until pale yellow and fluffy. Add the oil and the butter and mix until well combined. Mix in orange juice.
In a separate bowl, whisk the egg whites on high speed until stiff peaks form (2-4 minutes). Fold the egg whites into the batter using a rubber spatula, being careful not to over-mix and deflate the whites.
Pour batter into prepared pan and sprinkle with sliced almonds. Bake for 50-60 minutes or until a toothpick inserted into the centre comes out clean. Start checking the cake at 40 minutes: if it's becoming too brown on top, cover it with aluminium foil.
Cool in pan for 15 minutes, then invert cake onto a wire rack to cool completely. Once cool, dust top of the cake with confectioner's sugar and serve.