Preheat oven to 375°F (190°C). Line 2 baking sheets with parchment paper or a silpat mat. Set aside.
In the bowl of a stand mixer, fitted with the paddle attachment, add butter, granulated sugar, brown sugar and vanilla extract. Beat on medium speed for 8 minutes. Yes, 8.
Turn mixer off, add eggs one at a time, mixing on low speed until dough is smooth.
In a medium bowl, mix flour, baking soda and salt.
Add flour mixture to butter mixture all at once. Mix on low just until flour is blended, not a second more. This takes my Kitchen Aid mixer about 10 times around the bowl.
Add butterscotch chips and chocolate chips and mix for about 3 seconds, until chips are evenly distributed.
Using a 3 tablespoon scoop place dough balls 2 inches apart on baking sheets. Bake on center rack for 10-12 minutes, until lightly brown on the edges and still soft in the center. They may look underdone in the center, that's ok! Just don't over bake.
Allow to cool on baking sheet for 2-3 minutes, then transfer to cooling rack.