Preheat the oven to 350 degrees.
Prepare the tartlet crust by combining the flour, cocoa, salt and sugar in a medium bowl. Cut the butter into small cubes and add to the flour mixture. Using a pastry blender, cut the butter into into the flour mixture until it resembles damp sand. Make sure no remaining clumps are larger than a pea size. Beat the egg yolk with the cold water and slowly pour over the flour, stirring just until moistened. Form into a ball and cover with plastic wrap. Chill in the refrigerator for at least 30 minutes. Unwrap the dough, and roll out on a lightly floured surface. Using a round cutter, cut out circles and gently press into a tartlet pan with removable bottoms. My circles are about 5 inches (and the cutter came with the pan). Bake for about 8-10 minutes, until the crust has darkened. Transfer to a wire rack to cool.
On high speed with an electric mixer, cream the nutella, cream cheese and butter. Slowly add the sugar and continue to beat until fluffy. Stir in the vanilla. In a separate chilled mixing bowl, beat the whipped cream until soft peaks begin to form. Fold the cream into the nutella mixture. Once combined, pour into the cooled pie shells, cover with plastic wrap and chill for about 2 hours.
To make the chocolate ganache, heat the remaining whipping cream in a small saucepan- to a boil. Pour the hot cream over the chocolate bits in a heatproof bowl. Stir until the chocolate is smooth and completely incorporated. Allow to cool for about 30 minutes. Spread the ganache over the nutella tartlets and return to the refrigerator to cool for another 2 hours- or until the ganache sets. Top with a banana slice and crushed graham crackers.