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!).