Preheat oven to 375 degrees F. In a large mixing bowl beat butter and shortening with an electric mixer on medium to high speed for 30 seconds. Add the brown sugar, granulated sugar, baking soda, and salt. Beat until mixture is combined, scraping sides of bowl occasionally. Beat in eggs and vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Stir in chocolate pieces.
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