Grease an 8-inch (20-cm) springform cake pan.
Preheat oven to 375°F (190°C).
To make cake, beat egg yolks and sugar in a large bowl until pale and crearny.
Fold in hazelnuts, flour and almond extract.
Beat egg whites until stiff; fold into egg yolk mixture.
Turn batter into pan.
Bake cake 40 minutes or until a wooden pick inserted in center comes out clean.
Cool overnight on a rack.
To make filling and frosting, blend cornstarch, granulated sugar, a little milk and egg yolks.
Bring remaining milk to a boil.
Stir hot milk and vanilla into cornstarch mixture.
Return to heat and bring back to a boil.
Cook a few seconds, stirring constantly until thickened.
Cool, stirring frequently.
Cut cake into 2 layers.
Spread 1 layer with filling; top with second layer.
Knead almond paste to soften.
On a surface sprinkled with powdered sugar, roll out almond paste to a round large enough to cover top and sides of cake.
Press onto cake.
Blend powdered sugar with enough lemon juice to give a coating consistency.
Spread over cake.
Decorate with walnut halves.