Knead the flour, yeast, salt, sugar, milk and melted butter into a yeast dough. Let it rise in a warm place for about 30 minutes.
In the meantime, cut the leek into fine pieces. Fry this with the bacon in the pan until golden brown. Roll out the dough into two rectangles. Spread the leek mixture on top and roll up the whole thing. Cut pieces about 3 cm in size from the roll and place them upright next to each other in a springform pan or baking dish. Pre-bake at 75 degrees in the oven for 10 minutes. Then set the oven to 220 degrees and bake again for about 30 minutes (possibly shorter).
Eat warm or cold. Serve with cheese sauce if necessary.