A Boston bun is a large spiced bun

Spiced bun

A spiced bun is a bun to which spices were added during the making process. Common examples are the hot cross bun and the Jamaican spiced bun.-Spices used in spiced buns:Spices used in the preparation of spiced buns include:* cinnamon* mace...

 with a thick layer of coconut icing, prevalent in Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

 and New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

. Traditionally the bun contained sieved potato

Potato

The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...

, and modern versions sometimes contain raisin

Raisin

Raisins are dried grapes. They are produced in many regions of the world. Raisins may be eaten raw or used in cooking, baking and brewing...

s. It is often served sliced, to accompany a cup of tea

Tea

Tea is an aromatic beverage prepared by adding cured leaves of the Camellia sinensis plant to hot water. The term also refers to the plant itself. After water, tea is the most widely consumed beverage in the world...

. The origin of the name is unknown.

In New Zealand they're often called a Sally Lunn, especially in the North Island.