Walker's Doughnuts can be found at the end of Elizabeth Street, right across from the Flinders Street (train) station. The bright yellow sign can't be missed.
The store itself is quite small and only has minimal seating options available but they conveniently serve the doughnuts in a cardboard tray which makes them a perfect take-away treat.
The doughnuts themselves are amazing. The dough is so nice and soft and the flavors are just as good. They have such a vast variety of flavors, everything ranging from your traditional jam filled doughnuts to new-age flavors such as "classic Melbourne pineapple". They also sell coffees, American style hot dogs, milkshakes and 'old fashioned sodas'.
The store is very heavily old-school American-themed and so all the food is generous and hearty.
The price for these beauties are reasonable as well (deliciousness considered). I was thoroughly impressed with Walker's.