Preheat the oven to 400 degrees F, or heat the broiler. Butter the 6 ramekins, and put a pot of water on to heat for the pasta. While you're waiting for the water to come to a boil, toss the grated Gruyere with the cornstarch in a bowl, and chop the mozzarella and let it stand somewhere to lose any excess liquid.
Salt the water once it's boiling, and cook the pennette until on the firm side of al dente: read package instructions and start checking 3 minutes before the pasta's meant to be ready.
Meanwhile, heat the vermouth (or wine) in a saucepan big enough to hold the pasta later, and let it come to a boil before adding the chicken broth. Let it come to a bubble again, then take it off the heat and whisk in the cornstarch-tossed Gruyere. The cheese will melt into a mass of gooey cheese strings.
Add the mascarpone to the pan and whisk again, then add the truffle butter/paste or oil-go slowly and taste-stirring it into the sauce.
Tip the cooked, drained pasta into the sauce and stir to coat. Then tumble in the chopped mozzarella, and stir again so that it is distributed throughout.
Ladle the cheesy pasta into the ramekins, trying to get an even amount of pasta and sauce in each. The sauce will seem very liquid but don't panic, as the pasta sucks it up in the oven. Sprinkle the Parmesan on top, dividing it equally between the 6 ramekins, and give a good grinding of white pepper to each one. Don't worry if black pepper is all you've got. It's more a matter of aesthetics (mine) than taste.
Bake for 10 minutes in the hot oven, or broil until golden on top, and let stand for 5 minutes, at least, before eating.
Reprinted from the book Nigellissima by Nigella Lawson. Copyright (c) 2013 by Nigella Lawson. Photographs copyright (c) 2013 by Petrina Tinslay. Published by Clarkson Potter, a division of Random House, Inc.