Combine yeast and milk. Mix well until smooth and set aside.
In another bowl, combine flour, butter, sour cream, egg yolks, eggs, sugar and the yeast mixture. Mix well using an electric mixer at first, then using your hands. Knead on a floured work surface till smooth, adding a little more flour if the dough is sticking to your hands.
Form the dough into a long log. Slice into 5cm rounds, then cover with a cloth. Take one piece at a time and roll into a circle, about 5mm thick. Cut the circle into six triangles (like a pizza). Place a teaspoon of jam at the wider end of each triangle, then roll up like a croissant. Place on a baking tray lined with parchment. Repeat until all dough is used.
Bake the pastries for 20 minutes in the preheated oven, or till risen and golden brown. Allow to cool completely before icing.
For the icing, combine 300g icing sugar with a little water as needed to achieve a drizzling consistency. Drizzle the icing over the cooled pastries and enjoy!