PREHEAT oven to 350° F.
COMBINE brown sugar, 3/4 cup butter and water in large, heavy-duty saucepan. Cook over medium heat, stirring occasionally, for 3 to 4 minutes or until butter is melted. Add morsels; cook, stirring constantly, for 1 to 2 minutes or until smooth. Remove from heat. Add eggs one at a time, beating well after each addition. Stir in flour, baking soda and salt. Drop by rounded teaspoon onto ungreased baking sheets.
BAKE for 6 to 8 minutes or until edges are set. Cool on baking sheets for 1 minute; remove to wire racks to cool completely.
BEAT powdered sugar, 1/3 cup butter and vanilla extract in large mixer bowl. Add milk until smooth and of desired spreading consistency.
SPREAD about 2 teaspoons filling onto flat-side of 1 cookie; top with second cookie, flat-side down. Squeeze together gently. Repeat with remaining cookies.
FOR CHOCOLATE MINT SANDWICH COOKIES:
SUBSTITUTE 1/8 teaspoon peppermint or mint extract for vanilla extract in filling.
Makes about 4 dozen sandwich cookies.