a 10-by-15-inch baking dish (4 1/2 quart), a 9-inch springform pan, a pastry bag fitted with a small round tip
For the brownie: Preheat the oven to 350 degrees F. Line a 10-by-15-inch baking dish (4 1/2 quart) with foil leaving a 2-inch over hang on at least two of the sides. Coat the foil with nonstick cooking spray.
Whisk together the flour, cocoa powder and salt in a medium bowl. Melt the butter and semisweet chocolate in a medium saucepan over medium heat, stirring frequently, until smooth. Let cool slightly, about 10 minutes. Stir in the sugar and then the eggs. Add the flour mixture in two batches and stir to combine. Pour the batter into the prepared pan and bake until a toothpick comes out with just a few crumbs, 20 to 25 minutes. Sprinkle the bittersweet chocolate chips evenly over the top of the brownie. Let sit for 5 minutes to melt slightly. Use and offset spatula to spread the chocolate over the brownie. Let the cool completely in the pan on a cooling rack, about 1 hour. Refrigerate until the chocolate is completely set, about 30 minutes. Use the foil overhang to help lift the brownie out of the pan.
Lightly coat a 16-inch-long piece of parchment with nonstick cooking spray and place the brownie chocolate coating-side down (this helps to prevent the chocolate from cracking while cutting it). On one half of the brownie, trace the bottom of a 9-inch springform pan with a paring knife and cut out the circle. Fit the brownie circle, chocolate coating-side up, into the bottom of the springform pan and lock the side.
For the Jack-o'-Lantern eyes: From the remaining brownie half, use the paring knife to cut out two 2 1/2-inch equilateral triangles.
For the Jack-o'-Lantern nose: Cut one 2-inch equilateral triangle.
For the Jack-o'-Lantern mouth: Cut a 5-inch-wide half-moon. Cut out a bottom and top tooth that are slightly offset. (If the chocolate cracks at all use a hot, dry offset spatula to re-melt and smooth the chocolate.)
Make the cheesecake: Combine the gelatin with 1/4 cup water in a small microwave safe bowl and set aside to soften, about 5 minutes. Combine the cream cheese, pumpkin puree, confectioners' sugar, sour cream, vanilla and salt in a food processor and process until smooth.
Microwave the gelatin until melted, stirring as needed, about 30 seconds. Add the gelatin to the cream cheese mixture and process to combine, about 30 seconds. Pour the batter into the springform pan, on top of the brownie bottom.
Press the brownie eyes, nose and mouth into the batter until they are even with the top of the batter, taking care not to get any batter on top. Chill in the refrigerator until completely set, 4 hours and up to overnight.
For the pumpkin vine and stem: Mix the frosting with green food coloring in a small bowl until it is the color of a pumpkin vine (1 to 2 drops). Transfer to a pastry bag fitted with a small round tip and pipe a curly vine along the top edge of the Jack-o'-Lantern. Push the chocolate square into the top side of the cheesecake about 1/4-inch from the top to make the stem.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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