Ice Cream with OREO: Combine ingredients. Spread onto frozen half sheet pan. Freeze until solid. Remove from freezer and cut into 40 (3x1-1/2-inch) rectangles. Return to freezer until ready to use.
Crispy Rice Cookies: Melt marshmallows and butter in medium saucepan on medium-low heat, stirring constantly. Stir in vanilla.
Place cereal and OREO Pieces in large mixing bowl; add marshmallow mixture. Mix with hands until blended.
Divide mixture evenly between 2 half sheet pans generously sprayed with cooking spray. Spread mixture about 1/2-inch thick; press firmly. Let stand at room temperature 2 hours or until firm. Cut each pan into 34 (3x1-1/2-inch) rectangles.
To Assemble Sandwiches: Melt chocolate in saucepan on low heat until temperature registers 100°F.
Place 1 ice cream rectangle between 2 Crispy Rice Cookies. Dip each sandwich halfway into melted chocolate. Freeze until ready to serve.