Find grapefruit essence at bouletteslarder.com
. This recipe calls for 18 eggs and 9 lemons. The first 7 steps should be done the day before.To make caramel:
Combine cream and vanilla bean seeds in small saucepan. Cook over medium heat until hot but not boiling, about 2 minutes. Cover and set aside.
Combine sugar, 2 tablespoons water, corn syrup, and salt in a medium saucepan. Bring to a boil over medium heat, stirring to dissolve sugar. Boil, without stirring, until syrup is amber. Remove pan from heat and immediately add warm cream; mixture will bubble vigorously. Add lemon juice and whisk until blended. Let cool 5 minutes. Add butter and whisk until melted. Let caramel cool, whisking occasionally. Pour caramel into a small glass bowl; cover and refrigerate overnight.To make lemon cream:
In a glass or stainless steel bowl set over a saucepan of simmering water, whisk first 5 ingredients. Cook, whisking, until mixture thickens and reaches 180° on a digital thermometer, 15 to 20 minutes. Remove bowl and let cool to 140°, about 10 minutes, whisking occasionally. Whisk in butter, about 4 tablespoons at a time, until melted. Pour lemon cream through a strainer into a container and let cool. Cover and refrigerate overnight.To make lemon curd:
In a medium-heavy nonreactive saucepan, combine first 5 ingredients and grapefruit zest (if using instead of essence); whisk to combine. Add butter and cook over medium-low heat, stirring sides and bottom of pan constantly with a heatproof rubber spatula, until mixture thickens, coats back of spoon, and reaches 180° on a digital thermometer, about 15 minutes. Do not let mixture boil. Pour through a strainer set over a bowl. Stir in grapefruit essence, if using. Let cool; cover and refrigerate overnight.To make cake:
Preheat oven to 350°. Grease two 9" x 2" round cake pans. Line bottoms with parchment paper. Grease paper; dust pans with flour; tap out excess.
In a small bowl, mix 1/2 cup plus 2 tablespoons milk with egg yolks, egg white, zest, juice, vanilla, and salt with a fork.
In a stand mixer with paddle attachment, combine flour, sugar, and baking powder; mix on low speed until blended. Add butter and remaining 1/2 cup milk. Beat until smooth, scraping bowl. Increase speed to medium-high and beat 1 minute. Add egg yolk mixture, in thirds, beating on low speed after each addition until blended. Mix on medium until smooth, about 30 seconds. Add white chocolate and beat until well blended, about 1 minute, scraping bowl. Divide batter between pans and spread evenly. Bake 35 to 40 minutes, or until a toothpick inserted in center of cake comes out clean. Let cool 10 minutes in pans on wire racks. Run a knife around sides and invert cakes onto racks. Remove parchment; cool completely. Layers can be wrapped in plastic and refrigerated.To make lemon buttercream:
In a small saucepan, combine 1/2 cup sugar and 3 tablespoons water; bring to boil over medium-high heat. Boil until syrup is 250° on a digital thermometer, about 5 minutes.
Meanwhile, in a stand mixer with whisk attachment, beat egg whites and cream of tartar on medium-high speed until soft peaks form. Add 2 tablespoons sugar and beat until stiff peaks form. With mixer on medium-high speed, pour hot syrup in thin stream into whites, making sure syrup does not hit whisk. Scrape bowl and beat 1 minute. At medium speed, add butter 1 tablespoon at a time, beating until blended. If buttercream doesn't look well blended, beat on high speed until smooth, about 1 minute, scraping bowl. Add lemon cream, lemon juice, and grapefruit essence or zest and beat until smooth.To assemble cake:
In a small bowl, combine lemon juice and sugar; let stand 5 minutes and then stir until sugar dissolves. With a serrated knife, split cake layers in half horizontally, making 4 layers; turn cut sides up. Brush with lemon syrup. Microwave caramel on high 30 seconds; stir until spreadable; divide; spread on 2 layers. Place 1 layer caramel side up on a serving plate. Spread with half the lemon cream. Top with a plain cake layer, brushed side up. Spread with lemon curd. Add remaining caramel-topped layer; spread with remaining lemon cream. Top with last layer, brushed side down. Gently press down on layers. With an offset icing spatula or butter knife, smooth excess filling on sides. Refrigerate for 1 hour.
Spread about 1 1/4 cups buttercream over whole cake to crumb-coat. Refrigerate 20 minutes. Frost with remaining buttercream. Refrigerate; take out 30 minutes before serving.