Step 1

Preheat the oven to 325°F. Spray a 9X9-inch baking pan with baking nonstick spray. Line the bottom and 2 sides of the pan with parchment paper and spray again.

Step 2

In a medium bowl, sift the flour, cocoa powder, and cornstarch together. Add in the salt and set aside.

Step 3

Using a mixer with a whisk attachment, beat the sugar, eggs, and egg yolk on high speed for 5 minutes. It will become pale in color. While that mixes, melt the butter in the microwave and let it cool.

Step 4

Add the oil, melted butter, and vanilla and mix on low until combined. Add in the dry ingredients and mix on low until just combined. Pour in the chocolate chips and use a rubber spatula to fold it in.

Step 5

Pour the brownie batter into the pan. Melt the marshmallow fluff in the microwave for 20-30 seconds and mix. Use a 1 teaspoon to dollop the marshmallow fluff on top of the brownie batter. Use a butter knife to very light swirl the marshmallow fluff.

Step 6

Bake for 45-55 minutes. It is done baking when you stick in a toothpick, and it is slightly wet. If a lot of batter comes out, then bake for another 5 minutes.

Step 7

Cool on a cooling rack completely before cutting into them and serving.