In a large bowl with an electric mixer, beat the eggs, sugar, and vanilla, until the mixture is thick and pale. Sift the flour and baking powder over the egg mixture and fold them into it gently but thoroughly. Line the bottom of a 13x9x2-inch baking pan with parchment paper, spread the batter in the pan and bake in a preheated 400f oven for 8 minutes or until it is puffed and golden. Invert onto a lightly sugared kitchen towel. In a small saucepan, melt the jam over low heat. Stir in kirsch and spread the surface of the cake with the jam mixture. Starting with a long side and using the towel as a guide, roll up the cake, jelly-roll fashion and sprinkle it with Sherry. Wrap the sponge roll firmly in the towel. Let it cool and cut int into 3/4-inch slices. In a bowl, set over a saucepan of simmering water, beat the yolks and the sugar with an electric mixer until the mixture is thick and pale. Remove from heat and stir in gradually, milk, cream, and vanilla. Cook the custard in the bowl over the simmering water, until it registers 170f on a candy thermometer. Transfer the custard to a metal bowl and whisk in the creme fraiche and Sherry. Set the bowl in a larger bowl of ice water and whisk until custard is cool. Chill it, covering surface with plastic wrap, until it is cold. Assembly: In the bottom of a 2-1/2-qt. trifle bowl, scatter half the amaretti crumbs. Top them with half the sponge roll slices, and top those with the remaining amaretti crumbs. Sprinkle the Sherry over the remaining sponge roll slices, line the side of the bowl with the slices, and pour in the custard, spreading it to let it seep down around the sided and bottom. Chill the trifle for at least 2 hours and up to 24 hours. Just before serving, in a bowl with an electric mixer, beat the cream until it is stiff. Transfer it to a pastry bag, fitted with a decorative tip and pipe rosettes around the edge and in the center of the trifle. Sprinkle pistachios over the trifle and garnish the rosettes with the citron and small drops of jam. a 1989 Gourmet Mag. favorite