Genoise Layer Cake with Rum Syrup and Whipped Cream Frosting

Preheat oven to 350 degrees. Butter two 9-by-2-inch cake pans. Line bottoms with parchment paper; set aside. Whisk egg yolks and 1 cup sugar in the heatproof bowl of an electric mixer set over a pan of simmering water until sugar has dissolved and mixture is warm, 3 to 4 minutes. Attach bowl to mixer fitted with the whisk attachment. Add vanilla and salt. Beat on medium speed until the mixture is pale and thick, 3 to 5 minutes. Transfer to a large bowl.

Put egg whites into the clean, dry bowl of an electric mixer fitted with the whisk attachment. Beat whites on medium-high speed until soft peaks form. Gradually add remaining 1/2 cup sugar, beating until stiff, glossy peaks form.

Fold 1/3 of the egg white mixture into the yolk mixture. Fold in remaining egg white mixture. Fold in flour, then butter, until just combined.

Divide batter between prepared pans. Bake until a cake tester inserted into centers comes out clean, 25 to 30 minutes. Turn out cakes onto racks; remove parchment. Reinvert; let cool completely.

Make the syrup: Bring sugar and 1 cup water to a boil in a saucepan. Cook, stirring constantly, until sugar has dissolved. Stop stirring; bring to a boil. Reduce heat; simmer 5 minutes. Remove from heat, and stir in rum. Let cool completely.

Meanwhile, make the whipped cream: Put cream, sugar, and vanilla into the clean bowl of an electric mixer fitted with the whisk attachment. Beat on medium speed until medium peaks form.

Assemble the cake: Put pastry cream into the clean, dry bowl of an electric mixer fitted with the whisk attachment. Beat on medium-low speed until smooth. Trim tops of cakes to be flat. Halve each horizontally. Place 1 layer on a 9-inch cake round. Brush top with syrup; spread with half the pastry cream. Top with second cake layer. Brush with syrup; top with a 1/4-inch-thick layer of whipped cream. Top with third cake layer. Brush with syrup; spread with remaining pastry cream. Top with final cake layer, and brush with syrup. Insert a wooden skewer through center to hold layers together. Using kitchen shears, cut skewer flush with top of cake. Spread whipped cream all over cake. Refrigerate 1 to 4 hours. Decorate cake plate with leaves, if desired. Discard skewer after slicing cake.