• Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper, leaving overhang for easy removal. Clip the edges of the parchment with metal binder clips if needed and spray with cooking spray. Set aside.
• In a microwave-safe bowl, combine the butter and dark chocolate. Microwave in 20-30 second increments, stirring in between, until melted and smooth. Let cool slightly.
  ½ cup unsalted butter, ½ cup dark chocolate
• In the bowl of a stand mixer fitted with the paddle attachment, cream the granulated sugar, brown sugar, eggs, and egg yolk on medium-high speed for 4-5 minutes until pale and fluffy.
  ⅔ cup granulated sugar, ⅔ cup brown sugar, 3 large eggs, 1 egg yolk
• Add the vegetable oil and vanilla extract to the sugar mixture and mix on low speed until combined.
  ⅓ cup vegetable oil, 1 ½ teaspoons vanilla extract
• With the mixer on the lowest speed, slowly pour in the cooled melted chocolate and butter mixture. Mix just until combined; do not overmix.
• In a separate bowl, sift together the flour, cocoa powder, cornstarch, and salt. Mix to combine.
  ⅔ cup all-purpose flour, ¼ cup dark cocoa powder, 1 tablespoon cornstarch, ½ teaspoon salt
• Using a rubber spatula, gently fold the dry ingredients into the wet ingredients. Mix until just combined, being careful not to overmix. Make sure to get the bottom of the bowl.
• Pour the brownie batter into the prepared pan and smooth the top with a spatula. Bake for 28-32 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs. Do not overbake.
• Let the brownies cool in the pan for at least 30 minutes. While cooling, prepare the marshmallow fluff.

For the Marshmallow Fluff

• In the bowl of a stand mixer fitted with the whisk attachment, whisk the egg whites and salt on medium-high speed until frothy, about 3 minutes. Reduce speed to the lowest setting to maintain stability.
  2 egg whites, 1 pinch salt
• In a small saucepan, combine the sugar, corn syrup, water, and vanilla over medium heat. Heat until the sugar is fully dissolved and the mixture reaches 240°F (115°C), about 3-5 minutes.
  3 tablespoons granulated sugar, ½ cup light corn syrup, 2 tablespoons water, 1 teaspoon vanilla extract
• Turn the mixer back to medium-high speed and slowly pour the hot sugar syrup down the inside rim of the mixing bowl, letting it trickle down the sides to avoid scrambling the egg whites. This process should take about 2 minutes.
• Once all the sugar syrup is incorporated, increase the mixer speed to high and whip for 5 minutes, or until the mixture becomes stiff and shiny.
• While the marshmallow fluff is mixing, melt the dark chocolate in the microwave in 15-20 second intervals, stirring until smooth. Let cool slightly.
• Using a rubber spatula, gently fold the melted chocolate into the marshmallow fluff. Only fold 3-4 times to create a beautiful streaked effect.
  ¼ cup melted dark chocolate
• Spread the marshmallow fluff evenly over the cooled brownies. For an extra touch, toast the marshmallow topping with a kitchen torch, if desired.
• Slice the brownies into squares and serve. Enjoy your rich, fudgy brownies with their luscious marshmallow topping!