Cook chocolate, milk and corn syrup in large saucepan on low heat 5 min. or until chocolate is completely melted and mixture is well blended, stirring frequently. Bring to full rolling boil, on medium heat, stirring constantly. Boil 10 min., stirring constantly. Remove from heat. Add butter and vanilla; stir until butter is completely melted. Add to crumb mixture; mix well. Spread onto bottom of prepared pan.