Grease a round 8-inch (20-cm) cake pan.
Preheat oven to 350°F (175°C).
To make cake, put egg yolks and sugar into a large bowl.
Beat until pale and creamy, 5 to 10 minutes with an electric mixer.
Grate peel of both lemons.
Squeeze juice from one.
Add peel and juice to egg yolk mixture.
Grind hazelnuts to a powder; gradually fold into egg yolk mixture.
Beat egg whites until stiff; fold into mixture carefully with a metal spoon.
Turn batter into greased cake pan; smooth the surface.
Bake 40 minutes.
As soon as sides of cake begin to come away from pan, turn off oven and let cake stand 15 minutes in warm oven.
Turn out onto a rack to cool.
To make topping, beat cream with powdered sugar and rum in a medium bowl until stiff.
Spread whipped cream mixture thickly over top of cooled cake.
Decorate with Easter eggs.