Crumble the cake into a large bowl and work it with your hands until it is in small pieces. It should be in fine crumbs, but don't work it so much that it gets greasy and gummy.
Spoon three-fourths of the frosting into the bowl. I don't like to add all the frosting at once, in case the cake is very moist and the full can is unnecessary.
Stir the cake and frosting together until the mixture is well-combined. Toward the end, it might be easier to work it together with your hands, to make sure the frosting is evenly dispersed. It should be very moist and hold together if you squeeze a ball of cake between your fingers, but not too wet or greasy. If the cake mixture is still a bit dry, add more frosting to get it to the desired consistency.
To form the truffles, use a small cookie scoop or a tablespoon to portion out the cake into generous 1-inch balls, then roll the cake balls between your hands to get them round, and place them on a foil or waxed paper-covered baking sheet. Refrigerate them for 3-4 hours until they're cold and firm, or freeze them for about 45 minutes in the freezer.
After they have chilled and firmed up, take the purple velvet cake truffles out of the refrigerator or freezer and let them sit on the counter briefly while you melt the candy coating. They should be cold when you dip them, so they hold together, but if they're really frozen, that causes the candy coating to develop cracks as it hardens.
Melt the candy coating, and using forks or dipping tools, dip a truffle in the coating. Take it out of the coating and gently tap the fork against the lip of the bowl so extra coating drips off, then set the dipped truffle back on the baking sheet.
While the coating is still wet, you can also drizzle the tops with purple candy coating. Refrigerate the truffles to set the coating completely, about 20 minutes. Your purple velvet cake truffles are now ready to eat! These cake truffles are best served at room temperature, and they can be stored in an airtight container in the refrigerator for up to a week.