Rustic Bakehouse owner Katie Lamprey said she accidentally created the donut on September 14 after something in her choux pastry crackling did not set, so she mixed it with her husband's home made custard.

"A bakery on the mainland said they had invented the Caramilk donut and I'm like no, no, we had invented the Caramilk donut with a fail," she said.

"I thought Tassie needs to claim it...(and) it was funny how it came from a fail."

Mrs Lamprey can prove the country bakery was first to create the donuts, with her September 14 post showcasing them more than two weeks earlier than others.

Caramilk made its return to Australian shelves on October 2, with only a limited release in early 2018, after it was discontinued in 1994.

The earliest mainland bakery to create the Caramilk donut was Victoria's Daniel's Donuteria in Hoppers Crossing on October 1.

Two weeks before the Victorian bakery made its creation, Mrs Lamprey tagged comedian and Caramilk campaigner Christian Hull in her Facebook post about the donut she had accidentally created.

She later found herself making one to give to him at his Hobart show on September 22.

"One of my husband's sister's friends was going down to his show in Hobart," she said.

"She was like 'oh my god I've got a VIP (ticket) i'll get a donut down to him', and I was like no you can't, I've made it from a fail like I don't even know how i'm going to make it again."

The Cressy bakery owner said she remade it through her failed Croux attempts and Mr Hull ate it before describing it as the "most awesomeness thing he had eaten in his life".

Cadbury announced the return of Caramilk on September 23, more than a week after the Cressy bakery had created its first Caramilk donut.

Each day the Tasmanian bakery makes about 12 Caramilk donuts, half as long donuts and half as round donuts, in addition to a Caramilk cheesecake.