Heat the milk over low heat. Remove from the heat and leave it cool until you can put your finger into it.
Crumble the yeast into it, add the sugar then the butter. Mix, stir and leave for 5 minutes to infuse.
In a large bowl mix half the flour with the salt. Add the milk mixture, egg and egg yolk. Using an electric beater or by hand gradually mix into a soft dough by slowly adding the rest of the flour.
Knead it for 4-5 minutes them move the dough to greased bowl. Cover with a damp tea towel and leave to double in a warm place - about 60-90 minutes.
Grease and line a baking tray with baking paper.
Mix the sugar, cocoa powder & cinnamon.
Put the dough onto a well floured surface. Pinch down, then roll it out into a 2-3 mm thick rectangle.
Spread the melted butter then sprinkle evenly the sugar mix over it.
Starting with the long edge, roll up the dough (like a Swiss roll).
Using a sharp knife cut the roll into 15 mm thick slices.
Place the rolls, with the spiral side facing up, onto the tray with about a 1-2 cm gap between them. Sprinkle with some cinnamon.
Cover the tray with a damp tea towel and leave to rise again for about 30-45 minutes until the rolls are puffy. In the meantime preheat the oven to 175°C. Bake in the oven for about 20-25 minutes until the rolls are risen and golden brown, .
If you're short of time, don't leave it to rise but put the tray into the cold oven. Turn it up to 175°C. When when it reaches that temperature start to measure the 20 minutes baking time .
Allow the rolls to cool for 10 minutes before turning them out onto a plate.
Sprinkle some cinnamon & icing sugar over them (or, when still warm, brush them with sweetened milk).