To make the lemon sugar, combine the sugar and lemon zest in a food processor and blitz until the zest is broken down and combined.
Place the sugar on a plate to dry before storing in an airtight container in a cool, dry place.
To make the doughnuts, combine the yeast, warm water and a small pinch of sugar in a bowl and stir to combine. Rest in a warm place for around 5 minutes or until the yeast froths.
Add the warm milk to the yeast mixture.
In a large mixing bowl combine the flours, sugar, egg replacer and salt.
Using a spoon to stir (copying a bread mixture's action) slowing pour in the yeast mixture.
Add half the butter and knead in before adding the second half.
Dust a clean surface with flour and turn out the dough. Knead the dough for around 4 minutes. If the dough is super sticky, add more flour a small amount at a time (around 1 tablespoon). I added around 3 extra tablespoons.
Form the dough in to a ball and place in a lightly greased bowl covered with clingfilm. Place in a warm spot to rise for 1 hour.
Turn out the dough on to a floured surface and using a rolling pin roll it out to around 1.5cm.
Line a large baking tray with baking paper.
Using a small round cookie cutter or glass, cut out the doughnut rounds and place them on the baking tray. Cover loosely with cling film and set aside for 20 minutes.
Meanwhile, heat the oil in a heavy-bottomed pot or deep fryer to 170 degrees C.
Before frying, place kitchen paper on a plate or board to drain the doughnuts immediately after cooking.
In a mixing bowl add a good amount of the lemon sugar to toss the doughnut in.
Have another plate ready to receive the doughnuts.
Carefully place 3 or 4 doughnuts in to the oil. Cook on one side for around 1 minute or until golden. Carefully turn over the doughnuts with a large spoon and cook the other side.
Remove the doughnuts carefully and place on the kitchen paper for a minute.
Toss the doughnuts in the lemon sugar and pop on a plate.
Repeat with remaining rounds.
If you are filling your doughnuts, fill a piping bag with your lemon curd or jam filling.
Using the end of a teaspoon, make a small hole in the back side of each doughnut.
Fill each hold with filing before serving.