Bring a medium (6- to 8-quart) pot of water to a boil. Salt it generously (it should taste like the sea). Stir in the pasta and cook according to the package directions. Drain (but don’t rinse, or you’ll rinse away starches that will help thicken the sauce) and return to the empty pot.
Meanwhile, place a large (12- to 14-inch) sauté pan over high heat for several minutes. When hot, add the bacon and sauté until browned and crispy (lower the heat to medium-high if necessary to prevent scorching), about 5 minutes. Remove the pan from the heat and, using a slotted spoon or spatula, remove the bacon from the pan and drain on paper towels.
Pour off the fat from the pan (into a metal can or heatproof cup, not down the drain or you’ll potentially clog your pipes) and return the pan to medium-high heat. Add the heavy cream and bring to a boil, scraping up any browned bits from the bottom of the pan. Lower the heat to medium and continue to simmer until the cream is reduced slightly, about 3 minutes.
Add the Cheddar cheese and bacon. Stir well and cook over medium-high heat, stirring now and then, until the cheese has melted and the mixture thickens, about 3 minutes. Season with the salt and pepper.
Stir the sauce into the cooked and drained pasta in the pot. Place over medium heat and simmer for 1 to 2 minutes to thicken the sauce and allow the pasta to absorb the flavors, stirring now and then.
Serve in individual bowls topped with a dollop of sour cream and a sprinkle of sliced scallions.