Line a light-colored metal 9x13-inch pan with parchment paper, leaving an overhang to easily remove bars. Lightly grease with cooking spray. Don’t skip this step. Preheat oven to 350°F.
In a very large microwave-safe bowl, combine the 18 ounces chocolate chips and cubes of butter. Microwave in bursts of 15 seconds, stirring between each burst, until completely melted and smooth. (Be careful—microwaving too long risks burning chocolate.)
Remove from the microwave and immediately stir in the sugar. Add vanilla and one egg. Stir until completely incorporated. One at a time, add the remaining eggs and egg yolk, stirring with a wooden spoon after each addition until completely integrated. It’s thick, but avoid using a mixer, as it will beat in too much air. In a separate bowl, stir together the cocoa powder, cornstarch, and salt. Add to the brownie batter, along with the 1 and 1/2 cups chocolate chips. Stir until smooth. Transfer to prepared baking pan.
Bake for 27–33 minutes, watching carefully near the end. As soon as a toothpick comes out with moist crumbs (make sure it doesn’t hit a melted chocolate chip!) the brownies are done. This is almost always 29–30 minutes in my oven. Differences will occur based on individual ovens, humidity, etc. Remove and allow brownies to fully cool at room temperature before topping.
Optional Frosting: Add chocolate chips to a large, heat-safe bowl. Place the cream in a small, microwave-safe bowl and microwave on high for 1 minute. Pour over the chocolate chips. Cover bowl with the bottom of a cookie sheet or lid to a pan. Let stand for 4 minutes before stirring until smooth. (If chocolate isn’t melted completely, place in microwave for 15 seconds and stir again). Once smooth, pour over the completely cooled brownies and smooth until even. Chill, covered, for 1–2 hours or until firm. If frosting these brownies, store in the fridge as the frosting has a tendency to get melty.
Remove brownies from pan using the parchment-paper overhang. Cut with a sharp knife. For clean cuts, run the knife under hot water, dry, and make a cut. Repeat until all bars are cut. Return any leftovers to an airtight container in the fridge. Brownies are best served chilled.
Notes
Gluten free: These brownies are naturally gluten free—confirm with all packaging that products are processed in gluten-free environment.Nutrition Note: Nutritional figures do not include frosting.Storage: Store brownies airtight in the fridge for up to 5 days; unfrosted brownies can be stored at room temperature for 3 days. To freeze, wrap each brownie in plastic wrap, place in a plastic bag, and freeze for 2–3 months. Thaw at room temperature before eating.