Preheat oven to 325 degrees Fahrenheit (163 degrees Celsius). Line an 8x8" pan with parchment or foil, and set aside.
In a microwave-safe bowl, add the butter and sugar. Heat in the microwave on HIGH for about 1 minute and 15 seconds. Remove, stir, and cool on the counter for 5 minutes, then stir in the vanilla extract and cocoa powder.
In the bowl of a stand mixer, add the warm (but not hot) butter/sugar mixture. With the mixing speed on low, add one egg at a time, mixing just until incorporated.
In a medium-sized mixing bowl, whisk together flour, baking soda, cornstarch, and salt. With mixing speed on low, gradually add the dry ingredients. Mix until no flour pockets remain. Remove the bowl from the stand and fold in the chocolate chips and chunks.
Spread the brownie batter evenly into the prepared pan. Place in the oven and bake for 20-30 minutes or until the brownies are set. Remove from oven and allow to cool for 30-45 minutes before cutting and serving.
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Notes
*I love using a Ghirardelli baking bar for the chopped chocolate.- This recipe can easily be doubled to fit a 9x13" pan. The bake time might increase by 3-5 minutes. It will make approximately 24 brownies. I used a metal pan for these brownies, not Pyrex.**1/2 cup of cake flour can be substituted for the all-purpose flour and cornstarch.STORE brownies in an airtight container at room temperature to keep them moist and chewy; they will stay fresh for up to 4 days. To FREEZE, wrap them tightly in plastic wrap and then in foil. You can freeze it in your freezer for up to 3 months.To REHEAT, unwrap the brownies and microwave them on high for about 10-20 seconds, or until warm.