Combine flour, sugar, yeast and lemon rind in the bowl of an electric mixer fitted with a dough hook, mix to combine. Whisk together milk, buttermilk, eggs and 30gm melted butter and, with motor running, add to flour mixture, then mix on medium speed until dough is smooth and elastic (4-5 minutes). Shape into a ball, place in a lightly oiled bowl, cover with plastic wrap and stand in a warm place until doubled in size (1-1½ hours).
Meanwhile, for lemon ricotta filling, process ingredients (except cedro) in a food processor until smooth (1-2 minutes), transfer to a bowl, stir in cedro and refrigerate until required.
Preheat oven to 190C. Knock back dough, turn onto a lightly floured surface and roll to 5mm thick. Cut 12 rounds with a 7cm-diameter cutter and 12 rounds with an 8cm-diameter cutter (re-roll scraps if necessary). Place smaller rounds 5cm apart on oven trays lined with baking paper, place a heaped teaspoon of lemon ricotta filling in centre of each. Brush edges with milk, cover with larger rounds, press to seal edges well. Trim with a 7cm-diameter cutter. Cover and stand in a warm place until risen (1-1½ hours), then bake until bottoms are just golden (8-10 minutes).
Meanwhile, for lemon sugar, combine sugar and lemon rind in a bowl, spread on a tray. Dip hot doughnuts immediately in remaining melted butter, toss in lemon sugar and serve hot.