Heat oven to 350°F. Line two large cookie sheets with aluminum foil. In large bowl, combine oats, flour and baking powder. Add butter. Beat on low to medium speed of electric mixer until crumbly. Add powdered sugar, brown sugar, water and vanilla. Beat on low speed until dough forms. Divide dough in half. Place one-half on one cookie sheet; flatten with lightly floured hands into 13 x 9-inch rectangle, about 1/8 inch thick. Repeat using remaining dough and second cookie sheet. Bake 22 to 25 minutes or until golden brown, rotating cookie sheets after 12 minutes. Remove cookie sheets to wire racks. Sprinkle 1 cup dark chocolate pieces evenly over each large, warm cookie. Let stand 2 to 3 minutes. With spatula or knife, spread softened chocolate evenly over cookies. Sprinkle white chocolate pieces evenly over smooth chocolate on both cookies; let stand 2 to 3 minutes. With the tip of a knife, run through the white chocolate pieces, dragging them into the dark chocolate, making marble-like streaks. Sprinkle with gold dust, if desired. Cool completely. (Refrigerate 15 minutes to set chocolate, if necessary.) Remove large cookies from cookie sheet and peel off foil. Break each into 24 pieces. Store tightly covered in a cool place.