Preheat your oven to 350 F while you melt the chocolate and butter in double boiler using the heatproof bowl over a pot of simmering water on medium heat, 5 minutes.
Remove the chocolate from the heat and mix in the sugar, vanilla paste, and kosher salt until fully incorporated.
Slowly beat in the eggs until they are fully incorporated.
Mix in the flour, scraping the bottom to make sure it's all incorporated.
Line the pan with parchment and lightly butter it. Pour half the batter into the pan.
Place 5 tablespoons of Nutella onto the batter.
Gently draw your spatula through the Nutella to create a swirling pattern. Try to make it even.
Pour the other half of the batter into the pan and spread it evenly.
Drop the last 5 tablespoons of Nutella onto the batter.
Again draw your spatula through the Nutella, creating a marbled pattern and distributing it evenly over the brownie pan.
Bake on the middle rack for 30 minutes until a toothpick comes out with crumbs on it. Do not bake until the toothpick comes out clean.
Allow the brownies to cool for 20 minutes before cutting.