• Preheat oven to 350°F and line a metal 9×13-inch cake pan with parchment paper and spray lightly with cooking spray. 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.
• Melt 1 cup unsalted 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 beat 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 Dutch-process cocoa powder, and ¾ teaspoon salt into the butter and sugar mixture, stirring gently with a rubber spatula just until the dry streaks are gone.
  
• Fold most of the 8 ounces dark chocolate into the batter.
  
• Pour the brownie batter into the prepared baking pan, sprinkle the remaining chunks on top, and bake for 25 to 30 minutes.
  

Topping

• While the brownies are baking, prepare the icing by melting ⅔ cup salted butter in a saucepan over medium heat.
• Sift in 2½ cups powdered sugar and ¼ cup unsweetened cocoa powder and whisk until combined, then whisk in 3 ounces whole milk and 1 teaspoon vanilla extract.
• Remove from heat and allow to cool to room temperature.
• Spread the 16 ounces marshmallow fluff over the warm baked brownies, dropping by large scoopfuls and gently spreading with an offset spatula.
• Sprinkle 1 cup mini marshmallows over the top of the fluff and allow the brownies to cool for 30 minutes.
• Pour the cooled icing over the cooled brownies and fluff.
• Let the icing set completely before removing it from the pan and slicing it for best results. Use a plastic knife for the cleanest cuts. The fluff will stick to a metal chef's knife unless it's been sprayed with nonstick spray and wiped down.