To make dough, place all the dough ingredients in a large bowl, holding back about 40ml of water. Combine with your hands until the dough has come together and formed.
Slowly add remaining 40ml of water to the dough and knead in bowl, or using a stand mixer with dough hook attachment for about 10 minutes, until dough is smooth and elastic.
Place dough into a lightly greased bowl, cover with a damp cotton tea towel, leave in a warm place and leave to double in size for an hour.
Tip dough out onto a very lightly floured surface, and knock it back by kneading it a few times.
Divide the dough into 12 equal portions and roll each portion into a ball.
Place all balls onto a lined baking tray and allow to rise for an hour.
Preheat a deep fryer, filled with vegetable oil, to 180°C.
Gently lower each doughnut into the oil, cooking each side for about five minutes or until golden brown. Remove the doughnuts from the oil with a slotted spoon and immediately roll in sugar. Set aside and leave to cool.
When doughnuts are completely cool, using a small, sharp, knife make a cut into the top of doughnut, making sure you reach the centre.
Spoon some of the Lemon Curd into a piping bag, and pipe the filling generously into the doughnut. Top with lemon rind if desired.