Preheat the oven to 350°F and line an 8x8-inch metal baking pan (Note 1) with aluminum foil or parchment paper, leaving an overhang for easy removal.
Place the butter, semi-sweet chocolate, and 1/4 cup (74g) of the Nutella in a medium, microwave safe bowl. Microwave on high heat in 30-second increments until melted and smooth. Stop and whisk the ingredients together between each increment once they start to melt around the edges. Alternately, melt the chocolate mixture in a medium saucepan over medium-low heat.
In a separate, large bowl with an electric mixer or stand mixer fitted with the whisk attachment, beat the white sugar, brown sugar, eggs, and yolk yolk for 2-3 minutes, until it starts to foam and reaches the ribbon stage. When you lift the beaters out of the mixture it should leave small “ribbons” on the top of the batter that sink back in after a few seconds. Beat in the vanilla extract until combined.
Pour the melted chocolate mixture into the egg mixture and use a whisk or rubber spatula to combine, until no yellow streaks remain.
Add the flour, cocoa powder and salt and mix until just combined.
Pour the batter into the prepared pan. Melt the remaining 1/4 cup (74g) Nutella in small microwave safe bowl fro 30 seconds to soften it slightly. Dollop heaping teaspoons on top of the brownie batter and use a butter knife to swirl the Nutella into the top of the brownies. Don’t overdo the swirls, you want some large pools of Nutella on the top.
Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Mine took 38 minutes. Do not over bake.
Allow the brownies to cool completely on a wire rack. Once cooled, use the overhang of parchment paper to remove the brownies from the pan and cut into squares.