Peppery Pasta Carbonara with Poached Egg

Cook bacon in a 10-inch nonstick skillet over medium heat until crisp. Transfer bacon with a slotted spoon to paper towels to drain. Meanwhile, cook spaghetti in a 4-quart saucepan of salted boiling water until al dente. Reserve 1/3 cup cooking water, then transfer spaghetti with tongs to a small bowl, shaking off excess water, and keep pan of water simmering. Pour off all but 2 teaspoons bacon fat from skillet, then whisk butter into fat in skillet over medium heat. Add spaghetti, reserved cooking water, cheese, and a rounded 1/4 teaspoon pepper and cook, stirring, until sauce is thickened and almost completely absorbed, 2 to 3 minutes. Add bacon and 1 tablespoon tarragon and toss. Season with salt and pepper. Break egg into a cup and gently slide into water. Poach at a bare simmer to desired doneness (we prefer a firm white with a runny yolk, 2 to 3 minutes). Serve pasta topped with egg (transfer using a slotted spoon). Sprinkle with remaining tarragon if desired.