In a bowl, combine the milk with the vinegar and let it rest for 10 minutes.
In another bowl, combine flour, sugar, salt and yeast. Mix well, then add the oil and egg and mix well. Gradually add the milk-vinegar mixture.
Knead until a smooth dough forms. The dough will be very soft, so add a little more flour if necessary. Gather into a ball and place into an oiled bowl, cover and let rise till doubled in volume.
Preheat oven to 200 C / Gas 6.
Roll the dough into a long rectangle on a floured surface. Spread the dulce de leche over top, then roll up lengthways. Place in a greased 25cm tube cake tin. Brush the top with egg yolk.
Bake for 30 minutes in the preheated oven, or till golden and cooked through.
Dulce de leche
Dulce de leche is the Spanish name for what essentially is tinned caramel. You can make it yourself the same way you would boil a tin of condensed sweetened milk for banoffee pie. You can also buy it ready made; Carnation is one brand available.