1. Heat a large cast iron skillet or other heavy-bottomed pan over medium heat. Add bacon and cook for 2 to 3 minutes on each side, until as crisp as desired. Remove from pan and set aside on a paper towel to cool briefly. Chop into bite-size pieces.
2. Add Brussels sprouts and chopped scallion stems to pan with bacon fat and season with salt to taste. Cook over medium heat, tossing occasionally, until outer skins of Brussels sprouts have turned dark golden-brown and insides are al dente or fork-tender, about 12 to 15 minutes. Set aside.
3. Fill a large pot with 4 to 6 quarts of water. Add 1 teaspoon of salt and bring to a boil. Add pasta and cook according to package instructions until al dente. Strain out water.
4. While pasta is cooking, melt butter in a medium saucepan set over medium heat. Stir in 1 cup of milk and heat until steaming. In a separate bowl, whisk cornstarch into remaining 1 cup of milk until smoothly incorporated.
5. While stirring, slowly pour cornstarch mixture into heated milk. Bring to a simmer and simmer for 1 minute. Add Mature Cheddar, Crumbled Blue cheese, and smoked paprika and stir until cheese melts and sauce is smooth and slightly thickened.
6. Pour cheese mixture over noodles. Add bacon and Brussels sprouts mixture and toss until evenly combined. Serve immediately, with freshly ground pepper and chopped scallions if desired.