Majority of the cupcakes can be found across all the stages - (1 in each level of the stage - totally a 5 Cupcakes per stage).
But there is also 15 cupcakes that can be found on map and they are invisible, you need to come really close to see them. In each area there is 3 hidden cupcakes - They are not that hard to find, after few minutes of running around. But still, here are the locations.
They should be right where my pal is standing on each screen.
Port O' Potty