Grease a 9" tart pan and set aside. Heat the milk until lukewarm. Add yeast and stir, set aside for at least 5 to 10 minutes.
In a large bowl, combine the flour, salt, sugar, and lemon zest. Make a well in the center and add the egg, melted butter, and milk mixture. Stir until mixture comes together. Turn out onto a floured surface and knead for 10 to 15 minutes, until dough is smooth. Lightly oil a bowl, put in dough, and turn to coat. Cover and refrigerate for 30 minutes.
After dough has chilled, roll out dough on a floured surface to a large circle, 1/4" thick. Line the prepared tart pan with the dough. Refrigerate.
Preheat the oven to 400 degrees F. To make the filling, pour the milk in a medium saucepan. Split the vanilla bean in half, scrape the seeds from inside, and add seeds and bean to milk. If using extract instead, add extract to milk. Slowly bring to a boil over medium heat, then remove from heat and set aside to cool slightly.
In a medium bowl, sift the cornstarch and add a small amount of water to make a paste. Strain the milk into the cornstarch and discard the vanilla bean. Stir until smooth. Return mixture to saucepan and bring to a boil, stirring constantly. Cook for 30 seconds or until the mixture boils and thickens. Remove from heat and let cool.
In another bowl, beat together the sugar, cream cheese, lemon zest, and rum (or water). Add the cooled milk mixture and beat until well blended. Spoon the mixture into the dough-lined pan and smooth the top.
To make the streusel topping, in a small bowl, cut the butter into the flour until mixture is crumbly. Stir in the sugar. Sprinkle over filling.
Bake 20 to 25 minutes in preheated 400 degree F oven, or until golden brown. Cool tart in pan for about 10 minutes, then remove to a wire rack to cool completely.