Bring large pot heavily salted water to a boil. Add the penne and cook until pasta is just al dente (do not overcook).
Meanwhile, heat large sauté pan over high heat and add the olive oil. When oil is hot, add the pancetta and rosemary, and sauté until the fat on the bacon starts to turn translucent and very lightly brown, about 1 minute.
Add the garlic, chile and freshly ground black pepper to taste, and sauté until garlic and bacon turn richly brown, about 3 minutes.
Add the brussels sprouts, a large pinch of salt and a splash of water to pan, and sauté until sprouts just start to soften, about 2 minutes. Spread sprouts mixture in pan and press down to flatten. Let it sear for a minute, then stir it up and repeat. This helps brown the sprouts. Add the butter, and sauté for another minute.
Drain penne and add it to pan with brussels sprouts mixture. Cook, tossing, until everything is well mixed. Spoon into pasta bowls and top with a drizzle of oil and lemon juice, and a little cheese if you like.