Guanciale is similar to pancetta, but is fattier and more intense in flavor.
Bring a large pot of salted water to a boil. Prepare an ice-water bath. Cook brussels sprouts until just tender, about 10 minutes. Transfer to ice-water bath, and let stand until cool. Transfer to paper towels to drain.
Cook guanciale in a large skillet over medium-low heat, stirring occasionally, until fat is released and guanciale is crisp, 15 to 20 minutes. Season generously with pepper. Add half the brussels sprouts, and cook until just starting to brown, about 7 minutes. Season with salt and pepper. Transfer to a plate. Add oil and remaining brussels sprouts to skillet, and cook until just starting to brown, about 7 minutes. Season with salt and pepper, and combine with remaining brussels sprouts. Serve warm.