Chop the walnuts finely, but save a few for decorating the cake. Whisk the egg yolks and sugar with an electric beater, until the mixture whitens and is completly smooth. Add the chopped walnuts, the cornstarch and the rum. Beat the egg whites firm then fold them very gently into the batter. Butter and flour a cake pan. Pour the mixture and bake on a medium heat (350 F) for approximately 45 minutes. Unmold the cake on a rack and let it cool off. Make a pastry cream by mixing the cornstarch with the egg yolks and sugar. Boil the milk and melt the chocolate. Slowly pour over the egg mixture. Cook over the stove, stirring constantly with a spatula. Bring to a boil. Slice the cake in half horizontally, spread some of the cold chocolate cream on the bottom layer, then replace the top half of the cake. Cover the whole cake with the remaining chocolate cream. Decorate with halved walnuts.
