Cake: Preheat the oven to 190 degrees C. Grease a 18cm cake tin and dust with rice flour.
In a food processor mix 100g toasted hazelnut and demerara sugar. Once ground, mix in rice flour, cornflour, baking powder and ground ginger.
Melt the chocolate in a double boiler with the butter and the water. Stir to create a smooth mix, then remove from heat and cool slightly.
Separate egg yolks from egg whites then beat the yolks with the caster sugar until frothy. Add the vanilla. With a clean whisk, beat the whites to stiff peaks with a pinch of salt.
Stir the melted chocolate and the vanilla extract in the yolks bowl until smooth. Add the dry mix, stirring well, then incorporate the egg whites, gently, with a bottom-up movement, so they stay stiff. Pour the mix in the prepared tin.
Bake in preheated oven for 30 minutes or until a skewer inserted into the centre comes out clean and dry. Remove from oven, let slightly cook and transfer on a serving dish.
Glaze: Mix the icing sugar with the water. Chop roughly the remaining 50g of hazelnuts.
To Assemble: Brush the glaze onto the sides of the cake then the sides to about 2cm. Sprinkle the toasted hazelnuts on the glaze.
Dust the central top (the part not covered with the hazelnuts) with icing sugar. Serve.