Preheat the oven to 350°F. Lightly coat an 8×8-inch baking pan with cooking spray. Line with parchment paper, leaving an overhang on two sides to make removing the brownies easier. Lightly spray the parchment paper.

Place the chopped bittersweet chocolate and butter in a microwave-safe bowl. Microwave at 50% power for 30 seconds. Stir, then continue microwaving in 15–20-second intervals at 50% power, stirring between each interval, until almost completely melted. Stir until smooth and set aside to cool slightly.

In the bowl of a stand mixer fitted with the paddle attachment, combine 3/4 cup (222 grams) chocolate-hazelnut spread, sugar, and eggs. Beat on medium-high speed for 5 minutes, scraping down the sides of the bowl a couple of times.

Add the vanilla extract and mix briefly to combine.

Reduce the mixer speed to low and slowly pour in the melted chocolate-butter mixture. Mix until combined.

Gently fold in the flour and kosher salt with a rubber spatula until just incorporated. Do not overmix.

Transfer the batter to the prepared pan and spread evenly with an offset spatula.

Drop small dollops of the remaining 1/4 cup (74 grams) chocolate-hazelnut spread evenly over the surface. Using a toothpick, skewer, or butter knife, gently swirl the spread into the brownie batter, leaving distinct ribbons of chocolate-hazelnut spread visible on top.

Sprinkle evenly with flaky sea salt.

Bake for 25–30 minutes, or until the center is just set. A toothpick inserted into the center should come out with a few moist crumbs. Avoid overbaking to maintain the fudgy texture.

Allow the brownies to cool completely in the pan on a wire rack, approximately 1-1/2–2 hours.

Once completely cool, if time allows, place the pan in the refrigerator for 20–30 minutes.

Using the parchment paper overhang, lift the brownies out of the pan and transfer them to a cutting board. Cut into 16 to 32 squares, wiping the knife clean between cuts for neat edges.

Store brownies in an airtight container at room temperature for up to 4 days, refrigerate for up to 1 week, or freeze for up to 3 months.