First cook the macaroni to the instructions on the packet. Once the macaroni is cooked, drain it (don't rinse it). Place into a bowl with 15g of butter and toss though to stop it from sticking together. Cover and set aside.
Now in a saucepan add 60g of butter, 100g of sourdough starter and dash of the milk and stir over very low heat to form a roux. As it comes together and starts to cook, add a little more milk and start whisking. Get those arms moving!
Continue whisking over a medium heat as you add more of the milk, a little at a time, until all of the milk has been added. Continue whisking for around 5 minutes and you'll see the white sauce begin the thicken.
Once the sauce is thick and coats the back of a spatula, remove from the heat and add the shredded cheeses, mustard powder and garlic powder (if you're using them). Continue whisking until all of the cheese has melted through (it really doesn't take long).
Add the pasta on top of the cheese sauce and stir to coat all of the macaroni.
Now pour the macaroni cheese into an oven safe baking dish and top with the bread crumbs. Add dollops of butter (30g) all over the bread crumbs and bake in a 180C (350F) oven for around 30 minutes or until the bread crumb topping is golden and the sourdough macaroni cheese is bubbling underneath.
Once the sourdough mac and cheese is done, remove from the oven and allow it to sit for around 20 minutes before serving. This just allows it to cool slightly (so you don't burn your mouth), but also lets the sauce thicken up - believe me it's worth the wait!