Mix the flour, salt, sugar and yeast in a bowl. Add the butter and milk, mix and knead into a soft, smooth dough. Cover the dough and leave to rise at room temperature for approx. 1½ hrs. until doubled in size.
In a bowl, mix the ricotta with all the other ingredients up to and including the cinnamon, cover and chill.
To make the doughnuts
Halve the dough, roll out into two rectangles (each approx. 20 x 30 cm). Cut each rectangle into 6 squares. Place the filling in the centre of the squares, brush the edges with a little water. Fold the dough diagonally over the filling, press the edges down firmly with a fork.
Fill a pot to 1/3 with oil and heat it to approx. 160°C. Lower the doughnuts into the oil in batches using a slotted spoon, deep-fry for approx. 3 mins. on each side. until golden. Remove and drain on paper towels. Mix the sugar and cinnamon in a dish, toss the hot doughnuts in the sugary mixture.