Preheat your oven to 190 deg and combine the flour, sugar, yeast and lemon rind in the bowl of an electric mixer and mix to combine.
Whisk together the milk, yoghurt, eggs and 30 grams of the melted butter in separate bowl or jug and pour this into the flour mixture whilst the motor is running. Mix on a medium speed until the dough becomes smooth and elastic.
Shape into a ball, place in a well oiled bowl, cover with plastic wrap and a kitchen towel and let it stand in a warm place for about 1-1 1/2 hours (until it has doubled in size)
Once the dough has doubled in volume, turn it out on a clean bench, give it a good knocking and roll it out to 5mm thick. Cut 12 rounds with a 7cm round cutter and 12 rounds with an 8 cm cutter.
Place the smaller rounds spaced 5 cm apart on a grease paper lined tray and spoon in a teaspoon of your favourite jam filling.
Brush the edges with milk and cover with the larger rounds, pressing well to seal in the edges. (you can use the smaller cutter to trim the excess edges)
Cover again and let the doughnuts stand in a warm place for another 1 1/2 hours and bake in the oven for about 10-12 mins or until the bottoms and tops have browned.
Combine the lemon rind and sugar. As soon as the doughnuts come out of the oven dip into the remaining melted butter, toss in the lemon sugar and serve immediately.