While the crust is cooling, in a small bowl, whisk the cornstarch with 1/3 cup of the plant milk until no lumps remain. In a medium-sized saucepan over medium heat, whisk together the remaining milk, sugar, cocoa and salt. Once it starts to steam a little, slowly drizzle in the cornstarch slurry, whisking constantly to avoid developing lumps. Bring to a simmer and cook for about 2 minutes and the pudding has started to thicken (it will thicken further once chilled.) Whisk in the chocolate hazelnut spread and vanilla.