Preheat oven to 375°F.
Cook the bacon until crispy. Place on a paper towel to drain and cool. Reserve the bacon drippings and set aside. Once cooled, chop the bacon and set aside.
In a large pot, bring 4 quarts of water to a boil. Once boiling, add 2 Tablespoons kosher salt to the water. Add pasta and cook until al dente, according to package instructions.
Remove from heat. Drain pasta and return to pot. Toss pasta with 2 Tablespoons of the bacon drippings. Cover and set aside.
In a small saucepan, heat the milk. Do not bring it to a boil. Remove from heat and set aside.
In a large saucepan, melt 6 Tablespoons of the butter on medium low heat. Add the flour and cook for 2 minutes, whisking constantly. Add the hot milk and whisk together. Cook an additional 1-2 minutes, whisking until smooth.
Remove from heat and add the Gruyere and cheddar cheeses, the fine sea salt, pepper, and nutmeg. Stir until combined. Pour the cheese sauce over the cooked macaroni and gently stir to combine. Fold in the chopped bacon.
Fill a greased 3-quart casserole dish with the mac and cheese.
In a small bowl, melt the remaining 2 Tablespoons of butter. Combine with the breadcrumbs. Sprinkle over the top of the mac and cheese.