Whisk together the flour and salt. Cut in canola oil using pastry blender or two forks until crumbs and pieces of about 1/2” appear.
Mix water with vinegar, add by tablespoon until it begins to form a soft dry ball – do not over mix.
Turn dough out onto floured board and bring dough together. Divide in half. Use immediately or wrap each dough ball in plastic wrap and refrigerate until ready to use. Makes 2-9" crusts, so freeze one for another time.
Preheat oven to 425ºF. Roll out one half of the pie crust dough and lay into a 9-inch pie plate. Press into sides and turn under and create fluted edge. Prick shell many times with a fork. Blind bake on lower rack 12-15 minutes. Remove and allow to cool.
In a small bowl combine gelatin and boiling water, stirring until dissolved. Set aside.
In a medium saucepan whisk together sugar, cocoa, cornstarch and salt. Gradually add milk, whisking until smooth. Cook over medium heat, stirring constantly, until mixture thickens and comes to a boil, about 10 minutes.
Remove from heat and stir in milk chocolate, Nutella, reserved gelatin and vanilla. When melted and smooth, pour into baked pie shell. Press a sheet of plastic wrap against the entire surface. Refrigerate until very well chilled and set, at least 4 hours but preferably overnight.
To make whipped cream top, whip all ingredients with a mixer until stiff peaks form. Spread over pie. Top with a dusting of cocoa and chopped hazelnuts.