from "Viennese Desserts Made Easy" by Georgina Gronner:

Preheat oven to 350F/180 °C. Butter and flour a 10-inch springform pan. Set aside. Beat egg yolks and sugar until light and fluffy (about 5 minutes). Mix in almonds, chocolate and flour. Set aside. Beat egg whites until stiff. Fold in batter. Pour into prepared pan and bake in preheated oven until skewer inserted comes out clean (about 35 minutes). Remove from oven to cool on a wire rack. Prepare chestnut filling and set aside to cool.

When cake is cool, slice into two layers. Frost bottom layer with Chestnut Filling. Top with second layer and frost top and sides with remaining filling. Place a 4 inch wide jar lid or other round form over centre of cake. Sprinkle toasted almond slivers over top of cake. Remove lid. To serve, cut into 16 wedges.

Chestnut Filling: In a non-stick pan, cook egg yolks, cream, cornstarch and chestnut puree over low heat. Stir constantly until mixture thickens.

Remove from heat right away. Set aside to cool. Beat butter and sugar until light and fluffy. Add cooled chestnut mixture and beat until smooth. Taste and add more sugar or rum if desired.