Center a rack in the oven and preheat to 350 (F). Prepare an 8-inch tart pan with a removable bottom by spraying generously with cooking spray; set aside.
In a medium saucepan over medium-low heat, combine 1/2 cup unsalted butter and 1/2 cup Nutella until melted and combined. Remove from heat and add 3.5 ounces chopped dark chocolate, letting it sit for a minute or so to melt from the residual heat. Then whisk it all together to make a beautiful, glossy mess.
Add 1/3 cup granulated sugar, 1/4 teaspoon kosher salt, and 1 teaspoon pure vanilla extract and whisk to combine. Add 2 large eggs, one at a time, fully incorporating the first before whisking in the second egg. Sprinkle 1/2 cup all-purpose flour over the mixture and mix just until the flour disappears.
Pour the mixture into the prepared tart tin and bake for 25 to 30 minutes, until the top is nice and glossy and sort of crackled. It won't look 100% done when you take it out, but that means not having a dry hockey puck (see baker's notes — don't make my mistake!).
Cool completely before serving on a wire rack to ensure the best texture. If you want to eat it warm, maybe with vanilla ice cream, let it cool completely after baking, then pop it in the oven for 5 minutes at 350 (F) before serving.