Make the Lemon Curd:
In a medium saucepan, whisk together lemon juice, sugar, and egg yolks until smooth.
Cook over medium-low heat, whisking constantly, until mixture thickens and coats the back of a spoon (about 8-10 minutes).
Remove from heat and whisk in butter pieces one at a time until smooth.
Stir in lemon zest and salt. Press plastic wrap directly onto surface and refrigerate for at least 2 hours.
Make the Cake Layers:
Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large bowl, beat eggs and sugar with electric mixer on high speed for 8-10 minutes until thick and pale.
Sift together flour, cornstarch, and salt. Gently fold dry ingredients into egg mixture in three additions.
Combine melted butter, lemon juice, lemon zest, and vanilla. Fold into batter until just combined.
Divide batter between prepared pans and bake for 30-35 minutes until golden and springs back when lightly touched.
Cool in pans for 10 minutes, then turn out onto wire racks to cool completely.
Make the Lemon Cream:
In a large bowl, beat mascarpone until smooth and creamy.
In separate bowl, whip heavy cream and powdered sugar to soft peaks.
Gently fold whipped cream into mascarpone along with lemon juice, zest, and vanilla until combined.
Assemble the Cake:
Cut each cake layer in half horizontally to create 4 thin layers.
Place first layer on serving plate. If using limoncello, brush lightly over cake.
Spread 1/3 of lemon cream over layer, then dollop 1/3 of lemon curd and gently swirl.
Repeat layering process with remaining layers, cream, and curd.
Refrigerate assembled cake for at least 4 hours or overnight.
Before serving, dust with powdered sugar and garnish with lemon slices.