From Lorna Sass's book "The Pressured Cook". This recipe requires a 7-inch springform pan. It is a very rich, crustless, chocolate marbled cheesecake. Sometimes this cheesecake emerges from the pressure cooker looking "rustic" (with some peaks and valleys on top), but the flavor is dependably wonderful! This cheesecake will get firmer and denser when refrigerated overnight. For best flavor, bring to room temp before serving. This cheesecake freezes well. DO NOT SUBSTITUTE reduced fat products in this recipe, they contain more water and will not set up properly! Cooking Time includes 15 minutes for natural release. See NOTE for Lemon Cheesecake variation.