Powder coins were an April Fool's joke from 2017. We released an update to the game that you can download here: https://powdertoy.co.uk/Download/Build.html?ID=327

Nearly all elements + eraser tool were locked behind powder coins, which you gained at an incredibly low rate for playing. You could also "purchase" powder coins to get them more quickly. (It was a fake form, but it generally let you buy them, at the lower amounts).

You could also spend powder coins to promote any save you wanted to front page. This led to chaos as every powder toy player promoted random saves.

The online functionality (like promoting saves, or bonuses for inputting codes) are gone, so all that's left is the offline powdercoin acquisition.