Spread 350g Nutella onto a 8x8-inch pan lined with parchment paper. Freeze for 1 hour or until firm. Remove frozen Nutella with the paper onto a plate, and put back into the freezer (you only need to do this if you want to use the same pan for the brownies. If you have another pan, you can use it instead).
Preheat oven to 350°F/180°Line the 8x8-inch pan with parchment paper or aluminum foil, leaving enough overhang on the sides for lifting the brownies out after baking.
In a large heat-proof bowl, place chocolate, butter, and nutella and melt in the microwave in 20-30 second-intervals, stirring in between each interval, until melted (or, set it over a saucepan of simmering water, stirring occasionally). Add sugar and whisk until combined. Add eggs, one at a time, and whisk vigorously for a few times until well combined. Add flour and salt and fold, using a rubber spatula, just until mixed.
Pour half of the batter into the prepared pan and smooth the top. Remove frozen nutella from the paper and place on top. Top with the rest of the brownie batter.
Bake for 25-30 minutes. A toothpick insterted into the center of the brownie should come out with moist crumbs, but this will be a bit tricky to tell because of the liquid Nutella in the middle. If your baked goods are usually baked quickly then bake for 25 minutes, otherwise bake for 30 or even 35 minutes.
Refrigerate for at least 4 hours until set, then slice into 16 squares.
Because the brownies are fudgy, it’s better to keep them in the refrigerator, in an airtight container, for up to 3 days. They can also be frozen for up to 2 months. Thaw them overnight in the fridge.