Place the teaspoon of sugar, yeast and milk in a bowl, cover and leave to react for 20 minutes. The mixture should become frothy.
Meanwhile, sieve the flour and salt together, adding the egg, butter and yeast mixture. Mix well and knead until smooth. Cover and set aside in a warm place for 1 hour until doubled in size.
3) On a floured surface, knead the dough well then divide into 20 small pieces. Roll into flattened ball shapes, place a small amount of dulce de leche into each and carefully fold over, securing all of the edges to prevent the filling from escaping, and reshape into a ball. Set aside and repeat with the others.
4) Heat the oil in a deep saucepan until 150C (use a thermometer for this). Drop the balls in small batches for approximately 3 minutes, turning over halfway to ensure browning on both sides.
5) Prepare a sheet of baking paper near the oil and sprinkle with caster sugar and cinnamon. Lay out a piece of kitchen paper next to this. Once ready, use a slotted spoon to remove the balls and place on the kitchen paper to absorb any excess oil. Then transfer to the sugar paper and roll the bottoms in the sugar (avoid the tops). Leave to cool and repeat with your remaining balls.
6) Place the sugar and syrup in a pan over a gentle heat until melted. Turn up the heat and allow to bubble until a deep golden colour. Remove from the heat and pour in the cream, mixing well. Allow to cool slightly then dip the tops of the doughnuts into the caramel and place on a wire rack to cool slightly before serving.