Step 1In a large, heavy-bottom saucepan, whisk together the flour, sugar, lemon zest and eggs. Whisk in the lemon juice, then the milk. Heat the mixture over medium-high heat and cook, stirring constantly, until the custard thickens (it should coat the back of the spoon). Remove from heat and strain into a bowl set over a bowl of ice water. Stir until cool, then stir in the rum and vanilla. This makes a generous 2 cups of custard. Cover and refrigerate until needed.
Step 1In a large saucepan, combine the water and sugar over high heat. Bring to a boil, then simmer over low heat for 5 minutes. Remove from heat and stir in the rum. Cool completely.
Step 1In a large bowl, whip together the cream, mascarpone, sugar and vanilla until firm. This makes 4 cups whipped cream. Cover and refrigerate until needed.
Step 1Assemble the first layer of ladyfingers: Dip ladyfingers in syrup, one at a time, and place in a single layer on the bottom of an 8-inch square baking dish (or 10-by-8-inch dish). Cover the layer with half the custard.
Step 2Repeat with a second layer of ladyfingers, then top with the remaining custard.
Step 3Form a third and final layer of ladyfingers, then smooth over half of the whipped cream with mascarpone.
Step 4Refrigerate the tiramisu overnight to allow the flavors to be absorbed and marry. Serve each slice with a drizzle of chocolate sauce and a dollop of the flavored whipped cream, if desired.