Step 1

Preheat oven to 375°F.

Step 2

In a large mixing bowl, beat together the butter, oil, and sugars with an electric mixer.

Step 3

Add eggs and vanilla and beat until well blended.

Step 4

Whisk together the flour, baking soda, and cream of tartar and gradually add to butter mixture. Mix until combined.

Step 5

Drop by rounded teaspoonfuls onto ungreased baking sheets and bake until edges are lightly browned, 8-11 minutes.

Step 6

Remove to wire racks to cool. Enjoy!