Preheat your oven to 375°F (190°C) and grease a 13×18 inch baking pan.
In a large bowl, cream together the butter and sugar until smooth. Beat in the eggs one at a time, then stir in the vanilla.
Combine the flour, salt, baking soda, and cream of tartar; gradually stir into the creamed mixture.
Spread the dough evenly into the prepared pan.
Bake for 10 to 12 minutes in the preheated oven, or until light golden brown. Be careful not to overbake.
Allow the bars to cool completely in the pan before cutting into squares.
You can also add frosting or sprinkles on top for extra sweetness.