For the syrup, bring the sugar and water to a boil in a small saucepan. Cool and stir in the liqueur.
For the pastry cream, bring the milk to a boil with half the sugar. Whisk the egg yolks in a bowl with the salt and whisk in the remaining sugar. Sift the flour over the yolk mixture and whisk in smoothly. When the milk boils, whisk one-third of it into the yolk mixture. Return the remaining milk to a boil and whisk in the yolk mixture, continuing to whisk until the cream thickens and comes to a boil. Boil, whisking constantly to prevent scorching; for 1 minute. Scrape the cream into a clean, nonreactive bowl and press plastic wrap against the surface. Refrigerate until cold, about 2 hours. Before assembling the cake, gently stir in the liqueur.
To assemble the cake, cut the torta margherita into 3 horizontal layers using a sharp, serrated knife. Place one layer on a cardboard disk or platter and moisten with one-third of the syrup. Spread with half the pastry cream. Top with another layer, another third of the syrup, and the remaining pastry cream. Place the last layer on top of the filling and moisten with the remaining syrup. Cover the torta loosely with the plastic wrap while preparing the meringue.
For the meringue, combine the egg whites and sugar in the bowl of an electric mixer or in a heat-proof bowl. Place the bowl over a pan of simmering water and whisk the mixture gently until the egg whites are hot and the sugar has dissolved. Test a little of the mixture on your fingertip to see if the sugar has dissolved completely. Whip by machine on medium speed until cooled and increased in volume, 4 or 5 minutes.
Spread the meringue all over the outside of the torta. Use a pastry bag fitted with a
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