In a large mixing bowl, beat the butter, granulated sugar, and brown sugar until light and fluffy (about 2 minutes on medium speed). Add vanilla extract and eggs and continue beating at high speed for about 5 minutes longer, occasionally scraping down the sides of the mixing bowl. Your dough should be noticeably lighter in color and very fluffy.
In a medium bowl, mix together the flour, baking soda, cream of tartar, and salt.
Add the flour mixture to the butter/sugar mixture by the spoonful, mixing at low speed, and incorporating each flour addition until all have been combined.
Beat in the butterscotch chips and the toffee pieces, just until combined.
Preheat the oven to 350°F and line baking sheets with parchment paper or spray with nonstick baking spray.
Use a cookie scoop to form balls from the dough and space them 2 inches apart on a greased cookie sheet.
Bake for 9-10 minutes until the cookies are spread evenly across the top and are lightly golden brown around the bottom edges.
Transfer cookies to a cooling rack and let them cool.