Simmer jam, water, sugar, and lemon juice for 2 minutes, stirring until glossy.
Cool the syrup to room temperature, then stir in raspberry liqueur if using.
Whip the cold cream, confectioners’ sugar, and vanilla to medium peaks.
Loosen mascarpone with lemon zest, then gently fold into the whipped cream.
Briefly dip ladyfingers in cooled syrup. Arrange a snug layer in a 9x13-inch dish.
Spread half the mascarpone cream over the ladyfingers. Scatter half the raspberries on top.
Repeat with a second dipped-ladyfinger layer and the remaining cream and berries.
Cover and chill at least 4 hours, preferably overnight, to set and meld flavors.
Before serving, garnish with white chocolate shavings and crushed freeze-dried raspberries.