Prepare the sourdough: in warm milk, dissolve the sugar and yeast. Let the yeast activate for 15 minutes.
Melt the butter and let it cool.
Sift the flour into the bowl of a mixer, add sugar, salt, egg, butter and sourdough.
Knead the dough with a mixer for 5 minutes. (the dough is quite sticky, as it should be).
Cover the bowl with plastic wrap and send it to a warm place for 1 hour.
Put the dough on the table, dust it with flour and knead it well.
Divide into 8 parts and roll up the buns.