Preheat the oven to 350°F/175°C and line an 8x8 baking pan with aluminum foil.
Whisk the flour and salt in a bowl and set aside.
Melt the butter, you can do this either in the microwave using a microwave-safe bowl or over a double boiler. Once the butter is melted, either in the microwave or over the stove, add your chopped chocolate. Let the chocolate sit for a few minutes and then stir until it is completely melted. If the chocolate still hasn't melted, place the bowl back in the microwave and melt in 15 second increments. Allow the mixture to cool while you work on the next step.
In a stand mixer fitted with the whisk attachment, or in a large bowl with a hand mixer, whisk the oil, sugar, light brown sugar, eggs, egg yolk and vanilla extract for 6 minutes on medium-high speed.
Add your cocoa powder to the melted chocolate and fold until it is completely smooth. Pour the sugar mixture into the melted chocolate butter mixture and use a rubber spatula to fold everything together. Pour in the flour mixture and fold until there are no more streaks of flour.
Fold in most of the pecans and chocolate chips, but keep a small handful of each to the side.
Pour the brownie batter into the prepared pan. Dollop over the marshmallow fluff and use a toothpick or butter knife to swirl the brownie batter and marshmallow together. Sprinkle over your reserved pecans and chocolate chips.
Bake for 25-30 minutes, a toothpick inserted will come out with some crumbs.
Let cool completely before slicing into 9 or 12 bars.
Notes
Room Temperature: I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients about 2 hours before you start baking.Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Marshmallow Fluff: this can also be called marshmallow creme, marshmallow cream or marshmallow spread. Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.Storage: keep in an airtight container in the fridge for up to 5 days. You can freeze the bars for up to 3 months. Let them defrost in the fridge overnight before eating.