Pre-heat oven to 400ºF. In a pot of salted, boiling water, prepare the shell noodles according to package instructions. Drain and return to pot.
Add the stick of butter to the pot of drained shells, and stir until the butter is melted and the noodles are coated. Add ¼c of milk and stir.
Season with salt, pepper, and garlic, and stir until completely combined.
Add the shredded marble cheddar in increments, stirring and allowing it to melt, before adding more. Add another ¼c milk, and continue stirring/adding cheese until you add all 32oz of marble cheddar.
By this point, your cheese should be stretching like the pictures and videos above. If it's seeming dry or doesn't look creamy, add a splash more of milk. Add more salt/pepper/garlic if needed, and stir.
Once the shells are completely coated in cheese, and all the cheese is melted and stretchy, pour the mixture into a greased 9"x13" casserole dish. Top with the 8oz bag of sharp or mild cheddar.
Place mac n' cheese in the oven for 20-30 minutes, or until golden brown and bubbly. Once it's cooked, remove from the oven and let it sit for a few minutes. Enjoy!