Hi Barbara:-) Tartar Sauce is not what you substitute. What you need is Cream of Tartar which is a white powder found in the spice section of your grocery store. It is also a leavener for baking. If you do have it, you can try the following, which I have not tried, that I found on the internet.

Substitute for 1 teaspoon baking powder:
Combine 5/8 teaspoon cream of tartar plus 1/4 teaspoon baking soda

Also, you might make a cookie that doesn't call for baking powder. Here is my favorite brownie recipe that does not call for any baking powder and a recipe for Snickerdoodles that my girls love, which calls for cream of tartar and baking soda but no baking powder.

Quick and Easy Fudgey Brownies

4 oz. unsweetened chocolate, broken into pieces
3/4 c. butter or margarine
2 c. sugar
3 large eggs
1 1/2 tsp. pure vanilla extract
1 c. all purpose flour
1 c. chopped nuts (optional)

Heat oven to 350 degrees F. Grease a 13 X 9 X 2-inch baking pan. In a large microwave-safe bowl, place the chocolate and butter. Microwave at High power 1 1/2 to 2 min or until chocolate is melted and mixture is smooth when stirred. Add sugar; stir with spoon until well blended. Add eggs and vanilla; mix well. Add flour and nuts, if desired; stir until well blended. Spread into prepared pan. Bake 28 to 35 min or until a wooden pick inserted in the center comes out ALMOST clean. (Usually when brownies have started to pull away from the edges of the pan, they are done). Cool in pan on a wire rack. Cut into squares when completely cool. Makes about 24 brownies.

Snickerdoodles

1/2 c. butter or margarine, softened
1/2 c. shortening
1 1/2 c. sugar
2 large eggs
2 3/4 c. all purpose flour
2 tsp. cream of tartar
1 tsp. baking soda
1/4 tsp. salt
2 Tbl. sugar
2 tsp. ground cinnamon

Heat oven to 400 degrees F. Mix thoroughly the butter, shortening, 1 1/2 c. sugar, and the eggs. Blend in flour, cream of tartar, soda and salt. Shape dough by rounded teaspoonsful into balls. (Slightly smaller than a walnut).

Mix 2 Tbl sugar and the cinnamon; roll balls in mixture. Place 2 inches apart on ungreased baking sheet. Bake 8 to 10 min or until set and bottoms of cookies are light brown. Immediatelt remove from baking sheet and cool on wire racks. Makes about 6 dozen cookies.