Combine lemon juice, sugar, and water in a small saucepan over medium heat. Stir until sugar dissolves, about 2 minutes. Remove from heat and stir in lemon zest and limoncello if using. Let cool completely, about 20 minutes.

In a large bowl, beat egg yolks with 1/4 cup sugar until pale and thick, about 3 minutes on medium-high speed. The mixture should fall in a ribbon when you lift the beaters. Add mascarpone and beat just until smooth and combined, about 30 seconds. Do not overmix.

In a separate bowl, whip heavy cream with remaining 1/4 cup sugar, lemon zest, and vanilla until medium peaks form, about 2-3 minutes. The cream should hold its shape but still look slightly soft.

Gently fold whipped cream into mascarpone mixture in two additions, using a rubber spatula. Cut through the center and turn the bowl, rotating until no streaks remain. The mixture should be light and airy.

Working quickly, dip each ladyfinger into cooled lemon syrup for 1-2 seconds per side - they should be moist but not soggy. Arrange in a single layer in a 9x9 inch dish, breaking cookies as needed to fit. Spread half the mascarpone cream evenly over top. Repeat with remaining ladyfingers and cream.

Cover tightly with plastic wrap and refrigerate at least 4 hours, preferably overnight. The cookies need time to soften and the flavors to meld together.

Just before serving, dust generously with powdered sugar through a fine-mesh sieve. Add extra lemon zest if desired. Slice with a sharp knife, wiping clean between cuts for neat slices.