Preheat the oven to 350°F (180°C). Line an 8-inch square baking pan with aluminum foil, making sure to leave some overhang for easy removal. Spray the foil well with nonstick cooking spray (don’t skip this step or the brownies will stick to the foil) and set aside.
Combine the sliced butter and chopped semi-sweet chocolate in a large microwave-safe bowl. Microwave in 30 second increments, stirring well after each increment, until the mixture is completely melted and smooth.
Whisk in the granulated sugar until well combined, then whisk in ½ cup (150 grams) of Nutella until fully combined. Add the eggs and pure vanilla extract and mix until fully combined.
Add the flour and salt and continue mixing until just combined.
Scoop the batter into the prepared baking pan and spread it around into one even layer.
Add the remaining ¼ cup (75 grams) of Nutella to a small microwave-safe bowl. Microwave for 10 to 15 seconds and stir until smooth.
Drop spoonfuls of the Nutella on top of the brownie batter (I like to do 9 spoonfuls), then use a knife to gently swirl the Nutella into the batter.
Bake for 32 to 38 minutes or until the top of the brownies are set and a toothpick inserted into the center comes out clean with just a few moist crumbs.
Remove from the oven and allow to cool completely in the pan. Using the overhang from the foil, lift the bars out of the pan, and slice into pieces.