Step 1: Preheat your oven to 350°F (175°C).
Step 2: Beat together the softened unsalted butter, granulated sugar, and light brown sugar until light and fluffy (2-3 minutes).
Step 3: Add eggs and vanilla extract, and beat until well combined.
Step 4: In a separate bowl, whisk together all-purpose flour, baking soda, cream of tartar, and kosher salt.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined.
Step 6: Fold in butterscotch chips and English toffee bits until evenly distributed.
Step 7: Drop rounded balls of dough onto a lined baking sheet, spacing them about 2 inches apart.
Step 8: Bake for 10-12 minutes, until the edges are lightly golden.
Step 9: Let cool on the baking sheet for 5 minutes before transferring to a cooling rack.