Cream together margarine and brown sugar. Beat in eggs, peanut butter and vanilla. Add Flour, cornstarch, salt, baking soda and baking powder. Stir in Chocolate chips.
Roll into balls about 1 1/2 tsps onto an ungreased cookie sheet. Flatten slightly (they don't flatten much while cooking). Bake for 10 mins at 350 C. Makes 3-4 dozen.