In a food processor, combine the flour, sugar and lemon zest. Pulse the machine a few times to combine the dry ingredients and then add the cold butter. Continue pulsing the mixture until it looks like coarse cornmeal (a few larger pieces of butter are fine). In a liquid mixing cup, briefly whisk together the yolks, water and vanilla. Pour this mixture into the food processor with the machine running until the dough is just barely combined. Even if it looks crumbly, it should come together when pressed. Wrap the dough in plastic, flatten into a disk and chill for at least 2 hours.
On a lightly floured surface, roll the dough into a large, very thin rectangle, using the tart pan as a sizing guide. The dough should extend at least 3 inches beyond the width of the pan for best results. If the dough becomes too sticky while rolling, chill in the refrigerator until the butter firms up and then continue.
Carefully roll the dough around the rolling pin and gently unroll it into the tart pan. Press the dough evenly into the pan and then chill it in the refrigerator for 15 minutes.
Preheat the oven to 375 degrees F. Use a fork to prick numerous holes in the bottom of the dough to prevent air bubbles. Add a layer of aluminum foil and dried beans to weigh down the dough. Bake for 18 -22 minutes, removing the beans and foil halfway through, until golden brown. Allow to cool.
Prepare the pears:
In a medium saucepan, combine the water and sugar over high heat, stirring occasionally to help the sugar dissolve.
While the sugar syrup is heating up, peel, halve and core the pears.
Once the mixture is almost at a boil, add the vanilla bean, cinnamon sticks and pears. Cover with the lid slightly ajar and turn the heat to low. Simmer for 15-30 minutes, until the pears are soft.
If not using immediately, refrigerate the pears in their poaching liquid.
Prepare the mousse:
Heat the bittersweet chocolate and two tablespoons of heavy cream in a heatproof bowl set over a pot of simmering water and stir until smooth. Remove the pan from the heat and set aside to cool.
Add sugar and water to a small sauce pot. Cover and turn the heat to high. Once the liquid begins to simmer and steam has developed, remove the cover (this helps prevent crystallization). Using a candy thermometer, cook the sugar to the soft boil stage, 235-240 degrees F.
While the sugar syrup is cooking, whisk the eggs on high in a stand mixer with the whisk attachment until a soft peak has formed. Turn the mixer speed down to medium low and very slowly pour the syrup down the side of the bowl into the egg whites. Don’t pour the hot syrup directly into the meringue.
Whisk 1 cup cold heavy cream, preferably by hand, until it reaches a stiff peak. Fold into the cooled, melted chocolate. Once the whipped cream and chocolate are combined, fold the egg whites into chocolate mixture. Cover and chill until ready to use.
Assemble the tart:
Use a spatula to evenly spread chocolate mousse into the tart shell almost all the way to the top. Drain the pear halves and layer them on a bias, flat side down, across the length of the tart.
In a small bowl, microwave the chocolate in 30 second increments, stirring every time, until the chocolate is evenly melted. Drizzle over the pears.
Special Equipment: 14x5 inch rectangular tart pan with removable bottom Notes: The tart dough and poached pears can be prepared several days in advance.