Pre-heat oven to 350ºF. Lightly butter an 8 by 8 baking dish and set aside.

Cook the Macaroni: Bring a pot of water to a boil and cook pasta according to recipe directions, until the noodles are al dente (they should have just a little bit of bite to them). This is important because they will still be baked in the oven and soften more during that time. Strain and set aside.

Make the Sauce: To a blender, add whole milk, heavy cream and sourdough discard. Blend until the sourdough discard is completely combined with the milk, about 20-30 seconds. Set aside.If you don't have a blender, whisk together the milk, cream and discard until completely combined.

Shred the Cheese: Shred the sharp cheddar, monterey jack and gruyere cheeses using a cheese grater. I know this can seem like an extra step, but the pre-shredded stuff doesn't mix as well into the sauce (and nobody wants a grainy mac and cheese sauce!).

Make the Sauce: Melt 2 Tablespoons butter in a saucepan over medium-low heat. As soon as the butter is melted, pour in the milk/discard mixture. Whisk together. Continue whisking until the mixture constantly until it starts to thicken and bubble. The sauce will be thick enough to coat the back of a spoon. This takes about 5-7 minutes over medium-low heat. Remove the thickened sauce from the stovetop and stir in shredded cheeses, stirring until cheese is completely melted and incorporated into the sauce. Add mustard, hot sauce, salt, onion powder, garlic powder, parsley and pepper. Stir to combine.

Combine: Add the cooked macaroni noodles to the sauce and toss to coat them in the cheese sauce. Pour half the macaroni into the prepared pan. Sprinkle ¾ cup cheddar cheese on top of the noodles and then pour the remaining macaroni on top.

Breadcrumbs: Toss together melted butter, parmesan cheese and sourdough breadcrumbs until combined. Sprinkle breadcrumbs over the top of the mac and cheese.

Bake: Bake mac and cheese for 20-25 minutes until bubbly and the breadcrumbs are browning on top. Serve warm and enjoy.