Prepare Custard Cream. Prepare Coffee Zabaglione. Cream butter in a large bowl until pale and fluffy. Gradually add cooled zabaglione to butter, beating vigorously after each addition.
Refrigerate mixture 2 to 3 minutes to stiffen slightly. Whip cream and refrigerate. Preheat oven to 200° F. Put chocolate into a small ovenproof bowl and place in oven until chocolate has melted, 4 to 5 minutes.
Cut cake into 2 layers. Sprinkle rum over 2 cut surfaces of cake. Spread 1 layer with Custard Cream. Top with second layer. Spread zabaglione mixture evenly over top of cake. Hold cake in 1 hand and spread zabaglione mixture around sides of cake.
Gently press walnuts onto sides of cake. Put whipped cream into a pastry bag fitted with a medium star tube. Pipe a cream border around edge of cake. Decorate center of cake with 9 cream rosettes. Dip almonds into melted chocolate and place on top of rosettes.
Refrigerate cake until serving. Cake can be prepared up to 24 hours ahead. Let cake stand 30 minutes at room temperature before serving.
Preheat oven to 350° F. Butter and flour a 10-inch cake pan with a removable bottom; shake off excess flour. Melt butter in a small saucepan; cool slightly.
Put eggs, sugar and vanilla in a large bowl. Set bowl in a larger one containing hot water or use a double boiler off the heat. Beat at high speed 10 to 12 minutes or until mixture is pale and thick and has tripled in volume.
Sift flour over batter in several batches, gently folding in with a spatula after each addition. Gradually add cooled butter, folding gently until well blended.
Pour batter into prepared pan. Cook 20 to 25 minutes or until a wooden pick inserted in center of cake comes out dry.
Cool cake in pan 20 to 30 minutes, then transfer to a rack. Cake can be prepared 1 or 2 days ahead, covered with plastic wrap and stored at room temperature.
In a medium saucepan, bring milk just to a boil.
Put egg yolks, sugar and vanilla into a medium bowl. Beat 2 to 3 minutes until mixture is pale and thick.
Gradually beat in flour. Very slowly stir half the hot milk into yolk mixture. Pour mixture into pan containing remaining hot milk.
Whisk over medium heat 2 to 3 minutes or until custard has a medium-thick consistency. Do not let mixture boil. Place custard in bowl. Cover and refrigerate. COFFEE ZABAGLIONE: In a large bowl or the top part of a double boiler, beat egg yolk and sugar until pale and thick. Set bowl or top part of double boiler over simmering water; do not let water boil.
Gradually add coffee, beating constantly. Continue beating until zabaglione has doubled in volume and is soft and fluffy, 4 to 6 minutes.
Remove from heat and set pan or bowl over a bowl full of ice water. Stir with a whisk until mixture has cooled.Makes 8 to 10 Source: