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.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.