Grease 9-in. (23-cm) square baking pan; line with parchment paper, with overhang.
In medium saucepan, melt peanut butter and butter, stirring often, over medium-low heat. Stir in butterscotch chips and cocoa powder; cook 1 to 2 min. until chips are melted. Remove from heat. Let stand 20 min. until saucepan can be held comfortably. (The cooling helps the marshmallows retain their shape. If added right away into the hot mixture, they will melt or sink to the bottom of the pot.)
Fold marshmallows into peanut butter mixture. Scrape into prepared pan, smoothing top. Chill 1 hr. until set. Lift slab out of pan, peel off parchment paper and cut into 24 bars.