Preheat the oven to 375 degrees F.
In a food processor, pulse graham crackers until finely ground (to yield 2 cups). Add butter and pulse until crumbs are evenly moistened.
Transfer crumb mixture to a 9-inch tart pan with a removable bottom. Press firmly into the bottom and up the sides. Place tart pan onto a baking sheet; bake crust until lightly browned and set, 10 to 12 minutes. Remove from oven. Reduce the oven to 350 degrees F.
To make the fudge sauce, stir together cocoa powder, brown sugar, milk, salt and half of the chopped chocolate. Place in a small sauce pan and cook over medium heat, stirring, until chocolate is melted. Cook mixture at a low boil, stirring occasionally for 5 minutes. Remove sauce pan from heat, add remaining chocolate, butter and 2 teaspoons vanilla and stir until smooth. Any leftover sauce can be refrigerated in an airtight container, just reheat over low heat, stirring until melted or heat in microwave, on 30 second intervals, stirring frequently.
In a food processor or blender, combine the ricotta cheese, mascarpone cheese, granulated sugar, eggs, 2 teaspoons vanilla and the lemon zest + juice. Blend until smooth.
Drizzle the fudge sauce in the bottom of the prepared tart crust. Pour half the ricotta mixture over the chocolate and then drizzle the mixture with more fudge sauce. Add the remaining ricotta mixture and yeah....drizzle with more fudge sauce. Use a knife to swirl the fudge sauce into the ricotta mixture. Bake the tart in the preheated oven for 50 minutes to 1 hour or until just set. It is ok if the tart still has a little jiggle to it when it comes out. Set aside to cool.
To make the candied lemon, combine the sugar with ¾ cup water in a small saucepan. Bring to a boil over medium-high heat, stirring until the sugar dissolves, 3 to 4 minutes. Add the lemon slices and simmer until tender and translucent, about 10 minutes. Remove from the sugar and place on a parchment lined cookie sheet. Allow to dry before adding to the tart.
To serve, spread the tart with whipped cream and the candied lemons. Drizzle with any remaining fudge sauce. Store the tart in the fridge, but bring to room temp before serving.