Preheat the oven to 350 degrees Fahrenheit. Add the flour, confectioners' sugar, salt, and 3 tablespoons cocoa powder to a medium bowl. Whisk together.
Melt ½ cup butter, and pour the butter and the vanilla extract into the dry ingredients. Mix together until incorporated.
Press the dough into the bottom and sides of a 9-inch tart pan with a removable bottom.
Place the tart pan on a baking sheet and bake for 20 minutes. The crust will have gone from shiny to more of a matte brown color. Let crust cool completely, then remove crust from tart pan and set aside.
Meanwhile, to make the mousse, combine the instant pudding mix and the remaining 2 tablespoons cocoa powder together in a bowl. Add the heavy cream, and mix with an electric mixer on medium for 1 minute, until the mousse gets thick. Refrigerate until you are ready to use.
In a medium saucepan, combine the cranberries, water, and lemon juice. Cover and cook on medium-high heat for approximately 10 minutes, until the cranberries pop.
Pour the cranberry mixture into a blender or food processor, and process until smooth. Strain the cranberry mixture through a fine sieve and then pour the cranberry mixture back into the saucepan.
In a separate bowl, whisk together the egg yolks and the whole egg. Slowly pour the sugar into the bowl with the eggs, while continuing to whisk everything together. Pour the egg mixture into the cranberry mixture, whisking everything together in the saucepan until well-mixed. Heat the mixture on medium heat, whisking constantly until it thickens, approximately 10 minutes. Turn off the heat, and stir in the remaining 2 tablespoons butter.
Strain the mixture for a second time, cover, and refrigerate until the curd has cooled completely.
Spread the chocolate mousse evenly into the tart crust, then gently spread the cranberry curd evenly on top of the mousse. Refrigerate until the tart has set, about an hour, then serve.