Trim the cores on the Brussels sprouts and remove some of the outer leaves. Cut them in half, length-wise. Add to large mixing bowl and toss with 2 tbsp. of extra virgin olive oil, salt and black pepper to taste.
Pre-heat oven to 400 degrees F. Place Brussels sprouts flat on baking sheet lined with parchment paper. Bake for 30-45 minutes, checking frequently on the Brussels sprouts to ensure not to burn. Flip Brussels sprouts or toss baking sheet every 10-15 minutes to ensure even roasting.
While Brussels sprouts are roasting, bring a large pot of salted water to a boil. Add elbow macaroni and cook until just under al dente. Drain.
Grate all 4 cheeses and mix together in a large bowl.
In the same large pot, brown the pancetta in 2 tbsp. of olive oil on medium high heat. When browned, remove pancetta and about 3/4 of the excess fat and drippings. Retain 1/4 for cooking.
Add butter and 1/4 cup of flour to the pot, and whisk for 1 minute. Add 2 1/4 cups of milk to the pot slowly while whisking. Whisk until the mixture is completely smooth. If the sauce thickens too much, you can add more milk.
Turn the heat off or to very low. Add the cheese in handfuls to the sauce and continue to whisk. Keep about 1/4 of the cheese back (or grate additional cheese, depending on how cheesy you want the sauce to be) for sprinkling on top of the mac and cheese. Whisk until the mixture is smooth.
Add the macaroni, pancetta and Brussels sprouts to the sauce and coat evenly. Transfer the mac and cheese to a large buttered casserole dish.
Sprinkle the top with bread crumbs and additional cheese. Bake uncovered at 375 degrees F for between 15-30 minutes, until golden brown on top.
Remove from oven. Let cool, serve, and enjoy!