Prepare the pasta according to the packet instructions in a large saucepan.
Meanwhile, in a large frying pan fry the bacon until cooked through and crispy. To this, add the finely chopped leek.
Once they are both cooked, take the pan off the heat and keep to one side.
To make the sauce, melt the butter in a small saucepan and then whisk in the flour.
Mix until a smooth paste is formed.
Little by little add the milk, with the pan still on the heat, whisking after each addition and until the milk is all incorporated.
Add the yoghurt or crème fraiche and cheese and continue to stir until the cheese is melted.
Drain the cooked pasta and return to the saucepan, pour over the cheese sauce and toss well.
Spoon the macaroni cheese into serving dishes and top with the reserved fried bacon and leek, and scatter with breadcrumbs, extra cheese and parmesan.
Place the dishes under a hot grill for the cheese to melt and the topping to become nice & crisp.