For the starter dough, warm the 100ml of water and add the 10g of fresh yeast and the plain flour. Mix well and let prove.
When it has doubled in size fold it back into itself (knock it back) and add the ingredients from the second dough and let prove again.
Once the dough has doubled again knock it back and add the eggs and soft butter, knead the dough and let prove again. Once the mix has risen for the last time cut out doughnut shapes and cook at 180 degrees in a deep fat fryer or pot of oil.
When proving the dough it is best to do it in a warm place, think Goldilocks and the Three Bears! Too hot and the dough will over-prove and die, too cold and it won't activate and prove at all.
When removing the doughnuts from the oil, drain them on a little bit of kitchen paper then roll them in some caster sugar to satisfy a sweet tooth!