For a quicker version of this recipe, substitute store-bought pound cake or angel food cake and lemon curd.
For the cake: Preheat oven to 375 degrees. Coat two 18-by-13-inch rimmed baking sheets with cooking spray. Line with parchment cut to fit, and coat parchment with spray. Dust with flour, and tap out excess.
With a standing mixer fitted with the whisk attachment, beat egg yolks, 2 cups sugar, the milk, and vanilla on medium speed until pale and fluffy, about 4 minutes. Sift flour and salt over mixture in 3 additions, folding gently with a spatula. Transfer to a bowl.
Beat egg whites on medium speed until foamy. Add cream of tartar, and beat until soft peaks form. Add remaining 1/4 cup sugar, 1 tablespoon at a time, beating until stiff, glossy peaks form. Gently fold whites into yolk mixture.
Divide batter between prepared baking sheets, and smooth tops. Bake until golden brown and a toothpick inserted into center comes out clean, about 8 minutes. Let cool in pans on wire racks for 10 minutes. Invert cakes onto racks. Remove parchment, turn cakes right side up, and let cool completely.
For the lemon cloud: Fill a large bowl halfway with ice water, and set aside. Whisk together 6 eggs, 6 egg yolks, 1 3/4 cups sugar, the lemon juice and zest, and a pinch of salt in a heatproof bowl. Set bowl over a pan of simmering water, and cook, whisking constantly, until thick enough to coat the back of a spoon, about 15 minutes. Remove from heat, and whisk in butter. Strain through a fine sieve into a bowl. Set bowl in ice water, and let cool, whisking constantly. Place plastic wrap directly on surface of lemon curd, and refrigerate until cold, about 2 hours.
Put remaining 6 egg whites, cream of tartar, a pinch of salt, and remaining 1/4 cup sugar in a large, heatproof bowl. Clip a candy thermometer to side of bowl. Set bowl over a pan of simmering water, and whisk until mixture reaches 160 degrees and sugar has dissolved. Remove from heat. With a mixer on medium speed, beat egg white mixture until stiff, glossy peaks form, about 15 minutes.
Beat cream until firm (but not stiff) peaks form. Whisk lemon curd to loosen if needed, and then fold egg white mixture into chilled curd. Fold in whipped cream. Refrigerate until ready to use.
For the sherry syrup: Bring sugar, water, and sherry to a simmer in a saucepan, stirring until sugar has dissolved. Let cool completely.
Assemble cake: Place one cake on a cutting board, and brush with half the syrup. Brush 1 cup marmalade over cake. Top with second cake and remaining syrup and marmalade. Using a serrated knife, cut cake into 3-inch squares.
Place 5 or 6 squares upright in bottom of a large (14-cup or more) trifle bowl. Trim squares to fit curve of bowl so that the entire bottom is covered. Spoon 4 1/4 cups lemon cloud over top of cake squares. Repeat with two more layers of cake squares and lemon cloud. Drizzle with marmalade.
Sherry syrup can be refrigerated in an airtight container for up to 1 week.