1. Preheat the oven to 325°F (163°C). Lightly spray a 9×9-inch baking pan with nonstick spray. Line the bottom and sides with parchment paper, leaving an overhang for easy removal, and spray again.

2. In a medium bowl, sift together the flour, cocoa powder, and cornstarch. Add the salt and whisk until fully combined. Set aside.

3. Using a stand mixer or hand mixer with a whisk attachment, beat the sugar, eggs, and egg yolk on high speed for 5 minutes. The mixture should become pale, thick, and fluffy—this step creates the signature crackly brownie top.

4. While mixing, melt the butter in the microwave and allow it to cool slightly so it doesn’t cook the eggs.

5. Add the melted butter, oil, and vanilla extract to the egg mixture. Mix on low speed until just combined and smooth.

6. Gradually add the dry ingredients to the wet mixture. Mix on low speed until just combined. Do not overmix, as this can make the brownies cakey instead of fudgy.

7. Use a rubber spatula to gently fold in the chocolate chips, ensuring they are evenly distributed throughout the batter.

8. Pour the brownie batter into the prepared pan and spread it evenly.

9. Microwave the marshmallow creme for 20–30 seconds until slightly loosened. Drop spoonfuls over the brownie batter using a teaspoon.

10. Use a butter knife to gently swirl the marshmallow creme into the batter. Do not over-swirl; you want visible ribbons of marshmallow.

11. Bake for 45–55 minutes. Check doneness by inserting a toothpick into the center—it should come out with moist crumbs but not raw batter.

12. If the center is too wet, continue baking in 5-minute increments while monitoring closely.

13. Remove from the oven and allow the brownies to cool completely in the pan on a wire rack.

14. Once cooled, lift the brownies out using the parchment paper overhang. Slice into squares using a sharp knife for clean edges.