Preheat the oven to 180C / 350F. Grease donut pan well with butter (the batter should be making 12 donuts).
In a bowl, mix flour, salt, baking powder and sugar until combine.
Then, in a small bowl beat the eggs and vanilla extract.
Add the eggs mixture, greek yoghurt and melted butter to the flour mixture, mix well (the butter will be too thick).
Add water to the batter then add the sprinkles, mix (don't over stir).
Now, scoop the batter into the donut pan and bake the donuts for 10-15 minutes. Let them cool down.
Make the chocolate Frosting: In a small bowl, mix sugar powder, milk and cocoa powder well until combine. Dip the donuts into the chocolate frosting and scatter the sprinkles on top.
Notes
If you want to use chocolate instead of cocoa powder, you can microwave 100 g chocolate and 1 teaspoon butter for 1 minute, stir well then dip the donuts in it.