Line 13 x 9 x 2-inch baking pan with foil.
Melt peanut butter pieces and butter in small saucepan or bowl set over hot water until smooth. Spoon into large bowl. Whisk in sweetened condensed milk.
Gradually stir in crumbs and 1/2 cup peanuts (use hands if necessary to mix thoroughly). Pat the mixture evenly into the prepared pan. Refrigerate until firm, for about 30 minutes.
Melt chocolate pieces in small heavy saucepan set over very low heat, stirring occasionally, until smooth. Spread over top of peanut mixture. Sprinkle with remaining peanuts. Set aside until firm, about 3 hours. Cut into squares.