Pre-heat oven to 375°F and line a few baking sheets with parchment paper. (You can bake them at 350°F in a convection oven if preferred).

In a large bowl, whisk together the flour, baking soda, and cream of tartar. Set aside.

In the bowl of a stand mixer fitted with the paddle attachment, cream together the butter, oil, and sugars on medium-high speed until light and fluffy, scraping the sides as necessary.

Reduce speed to medium and add the eggs, one at a time, mixing just until combined.

Add the vanilla and mix until combined.

Reduce speed to low and add the flour mixture in three additions, scraping down the sides as necessary.

Using a scoop that holds 2 tablespoons of dough, drop batter onto the prepared baking sheet, spacing at least an inch apart.

Bake for 8 to 10 minutes, just until the edges begin to darken.

Let cool completely on the baking sheet before serving or storing.