For cookies, preheat oven to 350 degrees F. In medium bowl, combine oats, flour, and baking powder; set aside. In large bowl, beat 1 cup peanut butter, sugar, and butter until fluffy. Add 1/4 cup corn syrup, egg, water, and vanilla; mix until smooth. Stir in dry ingredients. Shape dough into 1 inch balls. Place on ungreased cookie sheets. Using bottom of glass dipped in sugar, press into 2-1/2 inch circles. Bake for 9 to 11 minutes or until light golden brown. Cool 1 minute on cookie sheet; remove to wire rack. Cool completely.

For filling, combine peanut butter and corn syrup; mix until smooth. Spread rounded teaspoonfuls onto flat side of half the cookies; top with remaining cookies. Sprinkle with powdered sugar and drizzle with melted chocolate if desired. Store loosely covered. Makes about 2-1/2 dozen cookies.