A light vanilla layer cake filled with bright, lemon mousse is "frosted" with toasted marshmallow-like meringue for a truly magnificent dessert.
Cake: Preheat oven to 350 degrees. Butter two 9-inch round cake pans; line with parchment. Butter parchment, then dust with flour, tapping out excess. Whisk together milk, eggs, and vanilla seeds.
In the bowl of a mixer fitted with the paddle attachment, beat together flour, sugar, baking powder, and salt on low speed until well combined. Continue beating while gradually adding butter until mixture is crumbly, about 3 minutes.
Slowly add half of milk mixture; increase speed to medium and beat until fluffy, about 2 minutes. Slowly add remaining half of milk mixture, scraping down bowl as needed. Add lemon zest. Beat until incorporated, about 1 minute more.
Remove bowl from mixer; divide batter evenly between prepared pans, spreading to edges using a small offset spatula. Tap pans on counter to release any air bubbles. Bake until golden brown and a tester inserted in centers comes out clean, 35 to 40 minutes. Transfer pans to a wire rack; let cool completely. Turn cakes out of pans and remove parchment.
Filling: Sprinkle gelatin over water; let stand until softened, about 5 minutes. Whisk together yolks, sugar, and lemon zest and juice in a small, heavy saucepan. Cook, whisking constantly, over medium-low heat, until mixture is thick enough to coat the back of a spoon, 8 to 10 minutes. Remove pan from heat; add gelatin mixture, stirring constantly, until gelatin is dissolved and mixture is slightly cool. Add butter a few pieces at a time, stirring well after each addition, until smooth. Using a rubber spatula, press filling through a fine sieve into a bowl. Cover with plastic wrap, pressing it directly onto surface to prevent skin from forming, and refrigerate until set, at least 2 hours and up to overnight. Stir filling; gently fold in whipped cream. Refrigerate 1 hour. Stir before using.
Using a serrated knife, split each cake horizontally into two equal layers. Place a cake layer, bottom-side down, on a serving platter. Spread 1 heaping cup filling evenly over it. Repeat process with two more cake layers. Top with remaining cake layer, bottom-side up. Refrigerate while you make meringue.
Meringue: Combine sugar, egg whites, and salt in the heatproof bowl of a mixer set over (not in) a pot of simmering water. Whisk until sugar is dissolved and mixture is warm to the touch and feels completely smooth when rubbed between fingertips, 2 to 3 minutes. Transfer bowl to mixer fitted with the whisk attachment; whisk on low speed until foamy. Increase speed to medium-high and whisk until stiff, glossy peaks form and mixture has cooled completely, about 10 minutes. Add vanilla, cornstarch, and vinegar; whisk until incorporated, about 1 minute more.
Assembly: Evenly spread top and sides of cake with a 1/2-inch thickness of meringue, smoothing with an offset spatula. Transfer remaining meringue to a pastry bag fitted with a 3/4-inch round tip. Position tip parallel to top of cake, about 1 inch from edge. Pipe meringue, using an even amount of pressure and dragging tip along surface of cake and down side of cake, on the diagonal, and working from right to left. Continue piping, rotating cake, to cover completely.
Using a kitchen torch, move flame back and forth along top edge of meringue until evenly browned. Top with raspberries just before serving.