In a medium bowl, cream together the margarine, peanut butter, and sugar. Beat in the eggs, one at a time, then stir in the vanilla. Combine the flour, baking powder and salt; stir into the peanut butter mixture. Drop by rounded spoonfuls onto the unprepared cookie sheet.
Bake for 8 to 10 minutes in the preheated oven. Do not overbake: cookies will be soft and chewy. Allow cookies to cool on baking sheet for 5 minutes before removing to a wire rack to cool completely.