Mix the flour, butter, salt and cheese together. Add the eggs, milk and yeast and knead for 5 minutes. As the dough is very sticky, it is best done with dough hooks, but if doing it by hand, work quickly to prevent the butter melting and be sure to flour your hands and the work surface. Leave it to rise in a warm place for 1 hour. Meanwhile, make the stuffing. Trim and clean the leeks, and slice them finely. Melt the butter in a pan and cook the leeks gently with a pinch of salt. Simmer for 3 to 5 minutes or until tender. When cool, add the cheese. When the dough has risen, keep about a third of it to one side. Butter a brioche or loaf pan and place the bulk of the dough in it. Make a hole in the centre at the top and fill with the stuffing. (The stuffing will sink a little, so it is best to place it near the top.) Cover with the remaining dough, which will expand to cover the top. Leave to prove for 30 minutes or until the dough has risen to the top of the pan.