"We normally bake one cheesecake each day but we have been doing four of these and they have been selling out every day.
"Because the chocolate is only available for a limited time, people want to try the cheesecake before the supply runs out."
The cheesecake sells for $8.50 a slice which customers have been happy to pay.
Former Whanganui mayor and foodie Annette Main endorsed the sweet treat on Facebook.
"Try it while you can, no one knows how long this limited edition chocolate will be produced for," she advised.
"We did and it's divine! As you can see, we weren't greedy and shared between three."
Cadbury, owned by US multinational confectionery company Mondelez, experienced strong demand in July last year for the caramelised white chocolate - discontinued in 1994 - when it released a limited-edition run of a million bars.
The latest limited edition Caramilk release was announced last Monday and blocks are being sold at Countdown, Pak'nSave, New World, The Warehouse and other confectionery retailers.
The recommended retail price is $4.69 and sales are limited to 12 per customer.
"We have been buying them from different outlets and trying to keep up with the demand," said Mr Watson.
There was no cheesecake left in the MudDucks cabinet by late morning on Sunday but patient customers would have the chance to sample slices of two new ones chilling in the fridge.
Mr Watson said staff are doing their best to see that no one misses out and MudDucks has a gluten-free version on the menu.
