In a large bowl, beat the butter, oil and sugars. Beat in eggs until well blended. Beat in vanilla. Combine the flour, baking soda and cream of tartar; gradually add to creamed mixture.
Drop by small teaspoonfuls onto ungreased baking sheets. Bake at 375° until lightly browned, 8-10 minutes. Remove to wire racks to cool.Yield: about 5 dozen.
Test Kitchen Tips
Salted butter and a splash of extract (try almond!) make these extra flavorful.
If you like your treats on the sweeter end of the spectrum, add a pinch of coarse sugar to the tops of these light and airy cookies.
Why cream of tartar? Baking soda needs an acidic ingredient to create the gas bubbles that make baked goods rise and lighten. Cream of tartar (aka tartaric acid) provides that in this recipe. Yay, chemistry!