Preheat oven to 375 degrees.

In a bowl, beat together the butter, oil, and both sugars.

Add eggs and vanilla, beat in until well combined.

Add in baking soda, cream of tartar, and flour. beat until just combined.

Using a cookie scoop, scoop dough onto an ungreased baking sheet. Bake at 375 for 8 to 10 minutes until lightly browned. Cool on wire racks. Makes about 5 dozen.