Boil pasta shells in salted water until al dente (about one minute less than the package directions.) Drain and set aside.
Melt butter in a medium dutch oven or sauce pan over medium heat. Whisk in flour and cook, whisking occasionally, for 2-3 minutes to cook out the flour taste.
Slowly whisk in the milk about 1/4 cup at a time until fully mixed in. It should resemble a thick sauce at this point. If it seems thin, cook for 2-3 minutes (whisking frequently) until the sauce thickens.
Repeat this step with the half and half. The sauce should be thick enough to coat the back of a spoon, and not run when you slide your finger over the back of the spoon.
Whisk in salt, pepper, garlic powder, mustard powder, and hot sauce until combined.
Turn the heat beneath the pan to "low". Whisk in the shredded cheese a handful (about 3/4 cup) at a time. Wait until the first handful of cheese is melted before whisking in the next.
Transfer the cooked pasta shells to the cheese sauce and toss to coat. Season with another pinch of salt and pepper, and serve immediately.
For Baked Mac and Cheese:
Preheat the oven to 350 F. Spray a medium-sized baking dish (about 2 or 3 qt. in size).
Transfer the mac n cheese to the prepared baking dish and top it with another handful of shredded cheese.
Bake at 350 F for 10-12 minutes until the cheese on top is bubbly and melted. Serve immediately.