PREHEAT oven to 350° F.
BAKE cookies according to package directions. Cool on baking sheet for 2 minutes; remove to wire rack to cool completely. Chop cookies into 1/2-inch pieces (about 2 cups).
LINE 8-inch-square baking pan with foil.
COMBINE morsels and sweetened condensed milk in medium, heavy-duty saucepan. Warm over lowest possible heat, stirring until smooth. Remove from heat; stir in 1 1/2 cups cookie pieces and vanilla extract.
SPREAD evenly into prepared baking pan. Sprinkle with remaining 1/2 cup cookie pieces, pressing in gently. Refrigerate for 2 hours or until firm. Lift from pan; remove foil. Cut into 48 pieces.