Place the yeast and 1 tablespoon each of caster sugar and flour into a medium bowl.
Gently heat the milk until lukewarm, add to the yeast mixture and stir well. Leave in a warm place for 10 to 15 minutes, or until starting to bubble.
Place the softened butter and remaining sugar and flour into a large bowl, then briefly rub in the butter.
Pour in the yeast mixture and stir with a wooden spoon until it comes together into a rough dough. Shape into a ball with your hands, adding a little more flour if it’s too sticky, then knead for a few minutes on a flour-dusted surface until smooth.
Place into a clean bowl, cover with a damp tea towel and leave in a warm place for 1 hour, or until doubled in size.
Once doubled, turn out the dough onto a flour-dusted surface, knock it back, then divide and roll into 12 equal-sized balls. Flatten slightly with your hand, then divide between 2 large baking trays lined with greaseproof paper.
Cover with a damp tea towel and leave in a warm place to prove for a further 45 minutes, or until doubled in size.
Place a medium pan over a high heat, pour in the oil and allow to heat up. To test if it’s hot enough, drop a piece of dough into the pan – if the dough floats to the surface, sizzles and turns golden, it’s about right.
Reduce the heat to medium then, using a slotted spoon, carefully lower the dough balls into the oil – they’ll need to cook in an even layer so you won’t be able to add all of them at once.
Fry for around 5 minutes, or until golden, turning halfway, then transfer to a double layer of kitchen paper to drain.
Meanwhile, warm the dulce de leche over a low heat until softened, then transfer to a piping bag with a small nozzle.
Combine the cinnamon sugar ingredients and ½ teaspoon of fine sea salt in a shallow bowl.
Insert the piping bag nozzle into the side of each doughnut and squeeze in the dulce de leche (you’ll know you’ve squeezed in enough when it starts to ooze out).
Roll the doughnuts in the cinnamon sugar, then tuck in.