Preheat oven to 400°F. Cook macaroni according to package and set aside.
In a large saucepan or Dutch oven, melt butter. Stir in flour, garlic powder and black pepper. Cook for about 1-2 minutes until well combined. Gradually add milk and cook until the mixture is smooth and bubbly, about 10 minutes. You'll want to stir constantly.
Remove sauce from heat and gradually add 4 cups of cheese and whisk until cheese is melted and smooth.
Add pasta and mix until everything is coated. Pour half of the mac and cheese into a 9x13 greased baking dish or 12 inch cast iron skillet. Add 2 cups of cheese evenly throughout and then the remaining mac and cheese. Top with remaining 2 cups of cheese and sliced jalapeños.
Bake for 20-25 minutes or until cheese is golden brown and bubbly. Remove from heat and let cool for 5 minutes before serving.
Notes
No overcooked pasta – Prepare the macaroni by cooking it al dente as per the instructions on the package. The pasta will continue to cook while in the sauce in the oven.
Freshly grated cheese – Avoid using pre-shredded cheese as it does not have the same delicious taste and may contain additional ingredients that are not necessary. It's best to grate the cheese yourself for optimal flavor.
Stir constantly – Once you've added the milk to the mixture, it's crucial to stir it constantly as it heats up and thickens. Failure to do so may result in burnt milk sticking to the bottom of the pot.
Lots of sauce – Who wants to eat dry macaroni and cheese? While preparing this dish, you may initially feel that there's too much sauce, but trust me, the macaroni will absorb it as it cooks, resulting in a creamy and delicious meal.
Create layers – You may have noticed that only half of the cheese goes into the sauce while preparing the mac and cheese. Start by adding only half of the cooked pasta to the baking dish, followed by two cups of cheese. Then, add the remaining half of the pasta and top with the remaining cheese