S’mores Cheesecake

• Position a rack in the center of the oven and preheat to 350°F. Lightly grease the inside of a 9-inch springform pan. Wrap the outside bottom and sides in two wide, overlapping layers of heavy-duty aluminum foil, keeping all seams above the eventual water line.
  
• Combine the graham cracker crumbs, sugar, melted butter, and salt until every crumb is evenly moistened. Press firmly and evenly across the bottom of the prepared pan and about 1 inch up the sides. Bake for 9–11 minutes, until the crust smells toasted and its surface looks dry. Transfer the pan to a wire rack and reduce the oven temperature to 325°F.
  
• Place the 6 ounces of chopped semisweet chocolate in a dry heatproof bowl. Melt it over barely simmering water, or in short microwave intervals, stirring until smooth. Let it cool until fluid but no longer hot.
  
• Beat the softened cream cheese on low to medium-low speed until smooth, scraping the bowl thoroughly. Mix in the sugar, sifted cocoa powder, salt, and vanilla until uniformly dark with no visible cocoa pockets. Add the sour cream and cooled melted chocolate, then mix on low only until evenly combined.
  
• With the mixer on its lowest speed, add the lightly beaten eggs in three additions. Stop mixing as soon as no yellow streaks remain. Scrape the bowl and fold the filling several times by hand to check the bottom and sides.
  
• Pour the filling over the crust and smooth the surface. Place the springform pan inside the roasting pan. Add enough very hot water to reach about halfway up the outside of the springform pan. Bake at 325°F for 60–70 minutes. The cheesecake is ready when the outer 2–3 inches are set while the center moves together as one soft area and does not ripple like liquid.
  
• Turn off the oven, crack the door open about 1 inch, and leave the cheesecake in the water bath for 1 hour. Remove the springform pan from the water, discard the foil, and place it on a wire rack. Run a thin knife around the upper edge only if the cheesecake is visibly attached to the pan. Cool to room temperature for about 1 hour.
  
• Cover the pan without touching the cheesecake surface and refrigerate for at least 5 hours, preferably overnight. Do not add the marshmallows before this chilling period.
  
• Place the 4 ounces of chopped semisweet chocolate in a heatproof bowl. Heat the heavy cream just until steaming, pour it over the chocolate, and leave it undisturbed for 2 minutes. Stir from the center outward until smooth. Remove the chilled cheesecake from the pan. Spread or drizzle the ganache over the top, leaving part of the cheesecake visible near the edge. Arrange the marshmallow halves cut-side down over the ganache. Toast them evenly with a kitchen torch until golden brown with a few darker spots. For the broiler alternative, place the unmolded cheesecake on a rimmed baking sheet 6–8 inches below the element and watch continuously.
  
• Add the broken graham cracker pieces after torching so they remain crisp. Let the finished cheesecake stand for 10 minutes before slicing. Dip a long thin knife in hot water, wipe it dry, and clean it between every cut.