- With the rack in the middle position, preheat the oven to 350°F (180°C). Line the bottom of an 8-inch (20 cm) springform pan with parchment paper. Do not butter the pan.
- In a bowl, combine the hazelnut flour, baking powder and orange zest.
- In another bowl, whisk the egg yolks with 6 tbsp (85 g) of the sugar and the vanilla with an electric mixer until the mixture is tripled in volume and forms a ribbon when falling off the beaters, about 5 minutes.
- In a third bowl, whisk the egg whites with an electric mixer until frothy. Gradually add the remaining sugar and whisk until soft peaks form.
- Add the dry ingredients to the egg yolk mixture. Add the melted butter and mix well. Gently fold in the egg whites with a spatula. Pour into the prepared pan.
- Bake for 40 minutes or until a toothpick inserted in the centre of the cake comes out clean. Turn the cake over onto a wire rack and let cool completely, about 2 hours.
- Pass a thin blade between the side of the pan and the cake. Unmould. Sprinkle with icing sugar and serve.
Hazelnut flour is available in some grocery stores, usually in the gluten-free aisle. If not available, you can make it yourself by finely chopping roasted hazelnuts in a coffee grinder or small food processor.