Leave butter on the counter at room temperature for 30 minutes or microwave on 50 percent power for 15-second intervals until you can press into it easily. Whatever you do, don't melt the butter! It won't behave the same in the batter.
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 on medium speed for 2 minutes, scraping sides of bowl occasionally.
Beat in eggs, corn syrup, and vanilla until combined. Just a splash of corn syrup ensures the chewiest, most tender cookies that stay fresh for days.
Beat in as much of the flour as you can with the mixer. Stir in any remaining flour, then fold in chocolate pieces.
Drop chocolate chip cookie dough by rounded teaspoons or a small cookie scoop 2 inches apart onto ungreased cookie sheets. Bake for 6 to 8 minutes or until edges are just light brown. Cool on cookie sheet for 2 minutes, then transfer to wire racks and let cool fully.
Pro storage tip: Layer cookies between waxed paper and make sure your container has a tight-fitting lid. Most cookie doughs can be frozen for up to 6 months. Just thaw in refrigerator before serving.