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 1.5 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.