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1
Beat the eggs and caster sugar together in a stand mixer at high speed until the mixture is pale, tripled in volume, and falls from the whisk in a thick, slowly dissolving ribbon. Sift the flour over the surface in two additions, folding each in with a large spatula using wide, deliberate strokes that preserve every bubble. Pour into a buttered and floured tin and bake at 170 °C until the surface is golden and springs back cleanly when pressed at the centre.
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2
Once cooled, cut the sponge into dome shapes using a hemispherical mould or a rounded bowl as a guide. A small hollow may be scooped from the flat base of each dome to receive filling cream.
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3
Dissolve the sugar in the lemon juice over low heat, then remove from the heat and stir in the limoncello. Allow to cool. Brush or spoon the syrup over the domes in two or three passes, allowing each application to be absorbed before adding the next. The sponge must be moist and yielding throughout but must hold its dome form without slumping — this is where the dish lives or dies; too little and the citrus fragrance never penetrates, too much and the structure collapses.
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4
Warm the milk to just below a simmer. Whisk the egg yolks and sugar until pale, then whisk in the flour. Pour the hot milk over the yolk mixture in a steady stream, return to the pan, and cook over medium heat, stirring constantly, until the cream thickens and loses all floury taste. Remove from the heat and grate the lemon zest directly into the hot cream; stir in the lemon juice. The cream should be glossy and coat the spoon thickly. Press cling film to the surface and cool completely.
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5
Melt the bloomed gelatin in a small spoonful of the warm pastry cream and fold back through the batch. Fold the whipped cream into the cooled, lemon-infused pastry cream in two additions until the mixture is homogeneous, pale, and holds a soft shape. The finished crema diplomatica should be light enough to fall slowly from a lifted spatula yet firm enough to pipe and hold a rosette.
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6
Spoon or pipe a measure of the lemon cream into the hollow at the base of each dome. Invert the dome onto a serving plate or wire rack. Coat the entire exterior with the lemon cream, piping the remainder in decorative rosettes or a spiral pattern over the surface. Refrigerate until the cream is set and the surface holds its shape without slipping.
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7
Remove from the refrigerator 10–15 minutes before serving. The delizia must be cold but not refrigerator-cold; at too low a temperature the lemon fragrance is muted. Consume on the day of assembly, or within 24 hours.