Whisk 5 large eggs, 1 ¼ cups granulated sugar, 1 cup lemon juice, and ⅛ teaspoon kosher salt in a medium saucepan until well combined. Cook over medium-low heat, whisking constantly and scraping the corners and sides of the pan occasionally, until thickened and the mixture coats the back of a spoon, 8 to 10 minutes. Do not let the mixture boil.

Remove the saucepan from the heat. Stir in 1 stick diced unsalted butter until melted and smooth.

Transfer to a bowl or jar and press a sheet of plastic wrap directly onto the surface to prevent a skin from forming. Let cool completely, about 2 hours. Meanwhile, make the lemon syrup.

Stir ½ cup granulated sugar, ½ cup lemon juice, and ½ cup water together in a medium saucepan. Cook over medium heat, stirring regularly, until the sugar is dissolved, 3 to 4 minutes. Pour into a shallow bowl or pie plate and let cool completely.

Beat 16 ounces mascarpone cheese and ¼ teaspoon kosher salt together with the whisk attachment in a stand mixer on medium speed just until smooth and soft, 15 to 30 seconds. Do not overbeat. With the mixer on medium speed, slowly pour in 1 ½ cups cold heavy cream and 1 tablespoon vanilla extract. Beat until soft peaks form, 3 to 4 minutes.

Gently fold in 1 cup of the lemon curd with a flexible spatula until no yellow streaks remain.

Working with 1 ladyfinger at a time, dip and submerge in the lemon syrup, then place in a 9x13-inch baking dish. Repeat, nestling them tightly together and breaking ladyfingers as needed to fit, until the bottom of the baking dish is covered in a single layer, about 20 ladyfingers.

Transfer half of the mascarpone filling (about 3 cups) over the ladyfingers and spread into an even layer. Repeat with a second layer of soaked ladyfingers and drizzle any remaining syrup over the ladyfingers. Spread the remaining mascarpone filling evenly over the top.

Cover the baking dish tightly and refrigerate overnight. Cover and refrigerate the remaining lemon curd.

When ready to serve, spread the remaining lemon curd over the top. Dust with powdered sugar if desired.