Preheat oven to 350°F and line a metal 9x13-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
Melt 1 cup salted butter and 2½ cup granulated sugar in a heavy-bottomed saucepan over low heat. Remove from heat and transfer to a large bowl, and let the mixture cool for 5 minutes, stirring often to release heat.
Add 2 tablespoons canola oil and 5 large eggs to the butter and sugar and beat them in with a rubber spatula until fully combined.
Add 1 tablespoon vanilla extract and mix until combined.
Sift the ¾ cup + 2 tablespoons all-purpose flour ¾ cup + 1 tablespoon unsweetened cocoa powder, and ¾ teaspoon salt into the butter and sugar mixture, stirring gently and minimally, until almost all the dry streaks are gone.
Fold in the ½ cup semi-sweet mini chocolate chips.
Pour the brownie batter into the prepared baking pan and bake for 30 to 35 minutes.
Remove from the oven and allow the brownies to cool completely before adding the next layers. Allowing the brownie to cool prevents the hot fudge sauce from tearing up the top of the brownie when spreading it.
Once the brownies have cooled, microwave the 1 (11.75 oz.) jar hot fudge sauce for 30 to 60 seconds to make it easier to spread. Then spread it over the top of the brownies.
Prepare the crunch topping by melting 10 ounces Reese's peanut butter chips in a large nonstick pot.
Once melted, stir in the ½ cup peanut butter and salt until fully combined.
Quickly stir in 4 cups corn flakes then immediately spread the mixture out over the top of the hot fudge.
Let the top layer set completely before removing the brownies from the pan and slicing them with a large sharp knife into squares. We like using these plastic knives because they yield clean slices!