• Preheat oven to 350°F and line a metal 9×13-inch cake pan with parchment paper. Make sure some of the parchment is over the edge of the pan for easy removal. This may require two sheets, one going in each direction – make sure the one that goes across the shorter distance is under the one that goes along the full length of the pan.
• Lightly coat the sides of the prepared pan with nonstick spray or oil; this helps prevent the marshmallow from sticking later on.

Crust

• Melt 16 tablespoons salted butter in a medium bowl, then stir in 2½ cups graham cracker crumbs, ⅓ cup granulated sugar, and ¼ teaspoon salt and combine until the crumbs are fully moistened.
  
• Spread the crust mixture out in he prepared pan and use your hands or the bottom of a glass to press the mixture into the bottom to make an even layer.
  

Brownies

• Melt 1 cup salted butter in a heavy-bottomed saucepan over low heat. Add 2½ cups granulated sugar and stir until smooth, the butter and sugar have started emulsifying, and no butter is separated from the sugar. Remove from heat and let the mixture cool for 5 minutes, stirring often to release heat.
  
• Add 2 tablespoons canola oil and 1 tablespoon vanilla extract to the butter and sugar and mix them in with a rubber spatula until fully combined and there's no separation between the ingredients (no oil resting on top).
  
• Beat in 5 large eggs one at a time, beating with the spatula or a fork after each one. Mix until fully incorporated.
  
• Sift 1 cup all-purpose flour, ¾ cup + 1 tablespoon unsweetened cocoa powder, 2 tablespoons milk powder, and ¾ teaspoon salt into the butter and sugar mixture, stirring gently with a rubber spatula just until the dry streaks are gone. You can also sift into a bowl first, which is easier if measuring with weighted grams, and then just add them to the pot.
  
• Fold the 8 ounces chocolate bar into the batter.
  
• Pour the brownie batter into the prepared baking pan and bake for 28 to 33 minutes.
  

Marshmallow Topping

• Whisk together 2 envelopes unflavored gelatin and ⅔ cup water in the bowl of a stand mixer. Let the gelatin sit to bloom while you prepare the rest of the ingredients.
  
• In a medium saucepan, combine 2 cups granulated sugar, ½ cup water, and ¼ teaspoon salt and bring to a simmer over medium-high heat. Attach a candy thermometer to the side or use a candy thermometer spatula (I love mine).
  
• Continue to cook, stirring occasionally, until the temperature reaches 225°F; which should take about 5 to 6 minutes. Once the mixture reaches this temperature, immediately add ½ cup light corn syrup; this will drop the temperature, but it will rise back up quickly.
• Continue to cook, stirring constantly, until the temperature reaches 238°F, about 2 to 3 minutes more. Once the mixture reaches this temperature, immediately remove the pan from heat.
• Turn the stand mixer on low speed (stir setting); while running, slowly pour the sugar mixture down the side of the bowl into the gelatin mixture. This is going to smell awful, like a wet dog, but I promise it tastes amazing - some serious chemical reaction magic happens here.
  
• Once you have added all the sugar syrup, increase speed to medium (6 on a KitchenAid mixer) for about 1 to 2 minutes, then to high (10 speed) and whip until the mixture becomes very thick, fluffy, and glossy, about 6 to 7 minutes. Add the 1 tablespoon vanilla extract and whip for 30 additional seconds.
  
• The marshmallow topping is done when it slowly falls back into the bowl from the whisk. After a few minutes, what's left on the whisk should start to firm up and have a firmer/airier marshmallow texture.
• Spread marshmallow cream out on the brownies so it’s relatively level.
  
• Let sit for 4 hours before cutting.
  

Cutting Brownies

• Cutting these brownies is a bit of a process to get them just right. Begin by heating a large chef knife under hot water and wipe it dry. Then spray with nonstick spray and wipe off so it's just a thin coating. You could also dip a paper towel in oil and wipe it on.
• Make one slice in the brownies, then pull to slide the knife back out (NOT UP), and repeat heating and oiling the knife between each slice. This allows the knife to slice through the marshmallow and brownies cleanly.
  
• I like to slice the brownies in half across; then lengthwise. Next, I cut each quarter of the brownie into 6 even squares.
  
• Pull the brownie squares away from each other slightly so they aren't touching, then use a kitchen torch to toast the top of the marshmallow. If you toast them while they are touching, the marshmallow melts back together, which makes it a little messy to pull them apart for serving.