Combine first 4 ingredients in large bowl. Add butter and vanilla; mix well. Add nuts; mix until blended.
Press nut mixture onto bottom of 15x10x1-inch pan sprayed with cooking spray.
Bake 15 to 18 min. or until lightly browned. Turn oven off. Let dessert stand in oven 5 to 8 min. or until golden brown. Remove from oven; cool completely.
Melt chocolate as directed on package; drizzle over dessert. Let stand until firm. Break into pieces to serve.