In large bowl, break up cake scraps to a fine crumb with electric mixer. Beat in buttercream, cocoa powder and rum. Mix until well combined. Add nuts; mix well. Shape into 1-inch balls. Refrigerate until firm, about 2 hours. Let stand 5 minutes at room temperature before dipping.
Melt Candy Melts according to package directions. Let cool slightly. Dip rum balls; place on parchment paper-lined sheet pan. Store in airtight container at room temperature. The flavor will become more intense over several days.
Makes 30 rum balls
* Substitute ¼ cup water and 1 teaspoon rum extract, if desired.