Preheat oven to 375 degrees. Line baking sheets with parchment paper.

In a large mixing bowl, beat the butter, oil, and sugars until combined. Beat in eggs and vanilla.

In another large bowl, whisk together flour, baking soda, salt and cream of tartar. Gradually add flour mixture to the butter mixture, beating until just combined. Fold in chocolate chips.

Drop dough onto prepared baking sheets. Bake for 8-10 minutes, or until edges and bottoms are lightly browned. Remove to wire racks to cool.

Store for up to one week in an airtight container.