You'll need a pastry bag fitted with a 1/2-inch star tip to create the decorative rosettes.
Begin making this cake two days before you plan to serve it.
For lemon curd: Whisk first 4 ingredients in medium metal bowl.
Set bowl over saucepan of simmering water (do not allow bottom of bowl to touch water). Whisk constantly until thickened and instant-read thermometer inserted into mixture registers 160°F, about 10 minutes.
Remove bowl from over water. Add butter; whisk until melted.
Transfer 2 cups curd to small bowl for spreading on cake layers. Reserve remaining curd for filling.
Press plastic wrap directly onto surface of both curds. Chill overnight. (Can be made 3 days ahead. Keep chilled).
For cake: Position rack in center of oven and preheat to 375°F.
Line bottom of two 9-inch-diameter cake pans with 1 1/2-inch-high sides with parchment paper (do not grease pans or parchment).
Using electric mixer, beat egg yolks and 7 tablespoons sugar in large bowl until mixture is very thick and slowly dissolving ribbons form when beaters are lifted, about 4 minutes.
Using clean dry beaters, beat whites in another large bowl until soft peaks form.
Add remaining 7 tablespoons sugar, 1 tablespoon at a time, beating until stiff and glossy.
Fold half of whites into yolk mixture, then sift half of flour and 1/2 teaspoon salt over and gently fold in until incorporated.
Fold in remaining whites, then sift remaining flour over and fold in just until combined, being careful not to deflate batter.
Divide batter between pans; smooth tops.
Bake until tester inserted into center of cakes comes out clean, about 15 minutes.
Cool in pans on racks.
Run knife around edge of pans to loosen cakes.
Invert cakes onto 9-inch-diameter cardboard rounds, tapping on work surface if necessary to release cakes.
Cut each cake horizontally in half (layers will be thin). Peel off parchment.
For syrup: Place sugar in small metal bowl. Add 1/2 cup boiling water; stir to dissolve sugar. Stir in lemon juice.
For filling and frosting: Beat whipping cream and sugar in large bowl until peaks form.
Add mascarpone to lemon curd in medium bowl; whisk until blended.
Fold whipped cream into lemon-mascarpone mixture.
Place 1 cake layer, flat side up, on platter.
Brush with 1/2 of syrup.
Spread 1/2 cup lemon curd over, then 1 cup lemon-mascarpone filling.
Top with second cake layer; brush with 1/2 of syrup and spread with 1/2 cup lemon curd and 1 cup lemon-mascarpone filling.
Repeat with third cake layer, syrup, lemon curd, and filling.
Top with fourth cake layer.
Brush with remaining syrup, then spread remaining lemon curd over.
Spoon 2 cups lemon-mascarpone filling into pastry bag fitted with 1/2-inch star tip (to be used for rosettes).
Spread remaining lemon-mascarpone filling as a frosting over sides of cake.
Pipe small rosettes of frosting over top of cake, covering completely.
Cover cake with cake dome; refrigerate at least 6 hours and up to 1 day.