Pour the flour into a big bowl and with your hands form a small hollow in the middle. Warm the milk. Mix 5 tbsp of the warm milk and 1 tbsp of the sugar with the yeast and pour into the hollow. Allow to rise for about ten minutes.
Now add 120g of the butter, the remaining milk and sugar, egg and salt and knead with your hands until you get homogenous dough. Form a ball, cover with a kitchen towel and let rise in a warm place free from drafts, for 40 minutes.
In the meantime mix starch, sugar and egg yolks. Halve the vanilla bean and scratch out the pith. In a big pot boil up milk, cream, vanilla pith and scratched out bean. Pour about 1/3 of the hot liquid into the egg-starch mixture; stir thoroughly, then pour this back into the original pot of milk-cream mixture and reheat. Stir thoroughly, until the sauce thickens. Remove from the stove.
When the dough has risen knead again on the worktop. Form a roll and cut off about 20 equally big slices (each about 50g).
Press them flat individually with the palm of your hand and put 4-5 blackberries in the middle. Press the dough rims together to form a small ball. Put the buns into a buttered oven-safe baking dish with the joined ends below. (upside down) Cover with a kitchen towel and let rise for another 15 minutes.
Melt the rest of the butter, gently brush the buns and bake golden-brown in the pre-heated convection oven for 30-40 minutes at 180°C. Let cool down and powder generously with powdered sugar. Serve with warm vanilla sauce.