Caramel shortbread
Caramel shortbread, also known as caramel squares, caramel slice, millionaire's shortbread, millionaire's slice, chocolate caramel shortbread, and Wellington squares is a biscuit confectionery item composed of a rectangular, triangular or  circular shortbread biscuit base topped with caramel and milk chocolate. Variations exist which substitute or add ingredients to cater to different tastes, dietary requirements or ingredient availability. History
Caramel shortbread is an adapted form of the original Scottish shortbread. Scottish shortbread originated around the 12th century, but its modern refined form is attributed to the efforts of Mary, Queen of Scots in the 16th century. The practice of adding additional topping or ingredients to shortbread to create unique variations dates back to at least the 19th century when candied orange peel and almonds were added. Where and when the combination of caramel and shortbread first appeared or was popularised is not known. Combinations of shortbread with caramel topping dates to at least the early 1950s. A recipe for Chocolate Caramel Crunch (Millionaires' Shortbread), with a caramel layer and chocolate top coat, was published in the Scottish Association of Young Farmers' Clubs book 'Recipes' in 1972. The recipes in this book were contributed by Scottish members of the organisation. Recipes for the dish with a chocolate topping also appeared in Australia in the early 1970s in the Australian Women’s Weekly. The earliest of these recipes appears in 1970 in an issue of The Australian Women's Weekly under the name ‘Caramel Shortbread’, but this version lacks the distinctive chocolate topping. Later recipes such as one from 1981 include the chocolate layer. The treat is still commonly associated with the cuisines of Australian, New Zealand and Scotland (as well as British cuisine more generally) and is still consumed in these countries regularly. In Scotland it is often served to the first visitor to a house on New Year's Day. It is also consumed around the world, where it is known by different names. In 2018 the dessert was featured in an episode of the American television show America's Test Kitchen. Naming
The name of the dessert varies by different regions and has varied throughout its history. This includes  the aforementioned names of caramel shortbread and chocolate caramel shortbread, as well as millionaire's shortbread, caramel slice, caramel squares, or millionaire's slice. The name "millionaire's shortbread" appears to  have originated in Scotland. The "millionaire" prefix to millionaire's shortbread or millionaires slice implies a level of decadence and wealth to the sweet treat, that it is an upgrade from regular shortbread. It could also be a reference to the caloric intake of the dessert. A Scottish saying warns consumers that "The millionaire’s shortbread is often richer in calories than the one who tastes [it] in hard cash". Variations 
Many different variations of caramel shortbread exist to cater to different tastes, dietary requirements, and ingredient availability. Some variants include using white rather than milk chocolate for the topping, adding espresso and hazelnut, substituting shortbread for  Anzac biscuits, adding raisins to the shortbread, or topping with peanut butter instead of chocolate. Some recipes adjust the recipe to cater to dietary requirements. Gluten-free and vegan versions exist. Some variations substitute ingredients to make the dessert more healthy. See also

 Twix
 Shortbread
 List of shortbread biscuits and cookies

References

Biscuits
Shortbread

Australian cuisine
Scottish cuisine
New Zealand cuisine
British cuisine

Australian desserts
Scottish desserts
New Zealand desserts
British desserts