Wiktionary
mix-in
n. 1 (context computing programming English) (alternative form of mixin English) 2 A dessert of ice cream with another ingredient mixed in.
Wikipedia
Mix-in
A mix-in is a type of dessert made of ice cream and another flavoring such as candy. Mix-in desserts are traditionally sold in an ice cream parlor and are made at the time of ordering. Popular examples of this dessert include Dairy Queen's Blizzard and McDonald's McFlurry.