Peel the squash with a vegetable peeler. Halve lengthwise, discard the seeds, then cut into 1-inch dice. Place in a large bowl and season with salt and pepper.
Heat the butter in a medium skillet over medium-high heat. When the butter ceases to foam and has turned a light brown, pull the pan off the heat and immediately add the sage, sugar, vinegar (stand back so as not to get splattered), molasses and Toasted Spice Rub. Mix well and let simmer over medium-low heat for 1 to 2 minutes to meld the flavors.
Pour the vinegar mixture over the squash and toss well, then transfer to a heavy rimmed baking sheet or baking dish large enough to hold the squash in a single layer. Place in the oven and roast, tossing at least once, until very tender and caramelized, about 1 hour. Set aside until cool enough to handle but still warm, so the liquids are runny.
Increase the oven temperature to broil.
Working in batches, if necessary, transfer the warm squash and all the cooking liquids to a food processor and process until smooth. Transfer to a buttered 1 1/2 pint gratin dish, or 6 to 7 (1/2 cup) ramekins, top with the crumbled biscotti and put under the broiler for 5 to 8 minutes or until biscotti is slightly toasted. You can also refrigerate for up to 5 days, or freeze for up to 2 months.
Variation for Smoky Butternut Squash: Cook the prepared squash on a baking sheet in a covered grill with soaked chips to give a slightly smoky taste. Substitute in any of the recipes that call for roasted squash.
If cooking kabocha, acorn, or other difficult-to-peel squash, cut in half, scoop out the seeds, and rub the insides and cut edges with the vinegar/molasses mixture. Place on a baking sheet, cut sides up, and roast at 400 degrees F until tender. Scoop out and puree.
Serve the puree on its own as a side dish for roast chicken, turkey, or pork; stir into polenta just before the end of cooking; use as a stuffing for ravioli; make into a soup; or use to flavor pastina. Or omit the sage, season with ground cinnamon and freshly grated nutmeg.