Several hours or day before serving, prepare Lemon Filling: In 2-quart saucepan, combine sugar, cornstarch, and salt until well mixed.
Gradually add water, stirring until smooth. Heat mixture to boiling over medium heat, stirring constantly with wire whisk; boil 1 minute (or until mixture thickens).
In small bowl, quickly stir some of hot mixture into egg yolks, then stir egg-yolk mixture into remaining hot mixture in saucepan until well blended.
Reduce heat to low; add butter and cook mixture for 5 minutes, stirring occasionally.
Remove from heat; stir in lemon juice and rind.
Cool to room temperature; cover surface of filling with plastic wrap and refrigerate until filling is completely cold-at least 2 hours.
Prepare Cake Layers: Heat oven to 375'F. Grease three 9-
inch-round baking pans.
Line bottoms of pans with parchment or, waxed paper; grease and flour paper.
In medium-size bowl, combine flour, baking powder, and salt.
In large bowl, with electric mixer at medium speed, beat 1 3/4 C sugar and very light and fluffy; beat in lemon juice, lemon rind and vanilla.
Add flour mixture alternately with milk to butter mixture, beginning and ending with flour mixture, beating until smooth batter forms.
In another bowl, with clean beaters and mixer at high speed, form.
With rubber spatula, gently fold egg whites into batter.
Divide evenly among prepared pans.
Bake 25 minutes, or until cake tester inserted in center comes Cool cake layers in pans on wire racks 10 minutes; invert layers onto wire racks, remove parchment paper and cool layers completely.
Meanwhile, prepare White-Chocolate Snowflakes: In double bars white chocolate.
Cover a baking sheet with aluminum foil. Spoon melted white chocolate into pastry bag fitted with writing tip.
Reserve pan in which white chocolate was melted.
Onto foil-lined sheet, pipe twenty-eight 2-inch snowflakes.
Decorate with silver dragees, if desired.
Place sheet of snowflakes in freezer until cake is completely frosted.
Prepare White-Chocolate Frosting: Melt remaining 2 1/2 bars temperature.
In large bowl, with electric mixer at medium speed, beat cream cheese until smooth.
Gradually beat in white chocolate, butter, and lemon juice until frosting is smooth.
To assemble cake, with sharp knife, trim tops of cake layers to make level. Place one cake layer on serving plate; spread with half of lemon filling; repeat with second cake layer and filling.
Place last cake layer, upside down, on top of filling to complete 3- layer cake.
Frost cake smoothly with White-Chocolate Frosting.
Remove snowflakes from freezer; gently peel snowflakes from foil.
Press a few snowflakes into side of cake; pile remaining on top of cake.
Serve immediately or store in refrigerator.
