Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the softened butter, granulated sugar, and brown sugar. Beat until light and fluffy.
Beat in the egg and vanilla extract until fully incorporated.
In a separate bowl, whisk together the flour, baking soda, salt, and cream of tartar. Gradually add to the wet ingredients, stirring until just combined.
Gently fold in the butterscotch chips.
Using a cookie scoop, portion out the dough and roll into balls. Roll each ball in sugar to coat.
Place dough balls on a baking sheet lined with parchment paper and bake for about 10-12 minutes.
Allow cookies to cool on the baking sheet for a few minutes before transferring to a wire rack.