Preparation

Bake the sponge cake:

• Position a rack in the middle of the oven and heat the oven to 350°F. Lightly grease a 9-inch cake pan and line the bottom with parchment. Sift together the flour, cornstarch, and 2 Tbs. of the sugar. Crack the eggs and yolk into the bowl of an electric mixer. Whip on medium speed, adding the lemon juice, salt, and the remaining 6 Tbs. sugar. Increase the speed to high and whip until the eggs are very fluffy, at least tripled in volume, and form soft peaks, 3 to 4 minutes. Remove the bowl from the mixer. Sift the flour mixture over the eggs in three separate additions, gently incorporating with a hand whisk each time. Put the melted butter in a small bowl, add a dollop of the batter, and stir gently. Add this to the mixing bowl, folding gently with a spatula to incorporate. Scrape the batter into the prepared pan. Bake until dark golden and springy when touched, about 40 minutes. Cool the cake in the pan on a rack.

Make the lemon mousse:

• In a heavy-duty, nonreactive saucepan, combine the egg yolks, sugar, lemon juice, and butter. Cook over medium heat, whisking constantly until the butter melts. Reduce the heat to medium low and stir constantly with a wooden spoon until the mixture thickly coats the back of the spoon, about 5 minutes. Strain into a clean bowl and stir in the zest and salt. Refrigerate, covered, until completely chilled. The mixture will be very thick. In a medium bowl, mash the mascarpone with a rubber spatula. Add a little of the chilled lemon curd and continue mashing until the mixture is lump free. Stir in the rest of the lemon curd, and then fold in the whipped cream. (If the mousse is too stiff, add a touch of unwhipped heavy cream to loosen it.) Refrigerate until it’s time to assemble the trifle.

Mix the soaking liquid:

• In a small saucepan, combine the sugar and water. Cook over medium heat, stirring constantly, until the mixture boils and the sugar has completely dissolved. Set aside. Pass 1 cup of the berries through a food mill fitted with a fine disk or force them through a fine sieve, mashing with a wooden spoon, into a medium bowl. Discard the contents of the strainer and stir the purée (you should have about 1/2 cup) into the sugar syrup. Add the lemon juice and set aside until it’s time to assemble the trifle.

Assemble the trifle:

• With a serrated knife, slice the cake into three equal round layers. (If you’re using wineglasses, cut the layers into squares the approximate size of the glasses.) Set a 9- inch-wide springform ring or cake ring that’s about 3 inches tall onto a flat serving plate. Put one cake layer on the bottom. With a pastry brush, moisten the cake well with the soaking liquid (it should be well moistened but not sodden).

• Spread on one third of the mousse and then arrange one third of the remaining 4-1/2-cups berries over the mousse. Place the second cake layer on top, moisten it with more soaking liquid, and repeat with another third of the mousse and another third of the berries. Do a third layering, ending with the remaining berries arranged on top (save the best-looking berries for this layer).
  

• Refrigerate the trifle until the mousse has firmed, at least 3 hours, but no longer than a day. Remove the cake form or ring just before serving and cut the trifle into slices like a cake.