Preheat oven to 180 degrees C (350 degrees F). Line a 18 cm (7 inch) cake pan with a circle of parchment paper.

In a bowl combine flour, cornstarch, ground hazelnuts and baking powder. Whisk gently and set aside.

In a large mixing bowl using an electric mixer beat together eggs and sugar until the volume increases significantly and the mixture becomes pale. Add oil, milk and vanilla and beat again shortly. Gradually add the dry ingredients, mixing gently with a rubber spatula, until absorbed.

Pour the batter into the prepared pan and bake for 40-50 minutes, or until a wooden skewer, inserted in the centre, comes out clean.

Let cool until room temperature before assembling the cake.

Heat milk and sugar at a medium temperature (in a saucepan on the stove or in the microwave). Whisk until the sugar is just dissolved: the mixture should be warm but not hot.

Trim any uneven parts from the top of the cake base. Place it on a serving plate (if you will be using a cake ring) or in a 22-23 cm (9 inch) springform pan. Sprinkle the syrup over the cake until all the liquid is absorbed.

Combine gelatin and water in a heatproof bowl and set aside for 5 minutes.

Whip the cream until soft peaks form. Using a rubber spatula, gently fold the ground hazelnuts into the cream.

Place the gelatin and the water on a double boiler (over a pan of gently simmering water). Whisk until the gelatin is dissolved. Add the white chocolate, piece by piece and whisk until it is melted. Let the mixture cool for 5 minutes and gradually add it to the whipped cream, carefully folding with the spatula.

Pour the mousse over the cake base in the springform pan (or in the plate with a cake ring extended around it). Refrigerate while preparing the chocolate mousse.

Whip the cream with the powdered sugar and vanilla extract until soft peaks form.

Using a rubber spatula carefully fold in the melted chocolate, until uniform. Pour the chocolate mousse over the hazelnut mousse in the cake pan. Smoothen on top with a spatula.

Refrigerate the cake for at least 8 hours before serving.

Decorate with chocolate candies, chocolate shaves and whole hazelnuts.