Preheat the oven to 350°F. Line a 10 × 15-inch baking pan with aluminum foil, leaving enough overhang to lift out the brownie later. Lightly coat the foil with nonstick cooking spray. In a medium bowl, whisk together the flour, cocoa powder, and salt.
Melt the butter and chopped semisweet chocolate in a saucepan over medium heat, stirring until smooth. Let cool for about 10 minutes. Stir in the granulated sugar, followed by the eggs. Fold in the flour mixture in two additions. Spread the batter evenly in the prepared pan and bake for 20–25 minutes, until a toothpick inserted into the center comes out with a few moist crumbs.
Sprinkle the bittersweet chocolate chips over the warm brownie. Let them sit for about 5 minutes to soften, then spread the melted chocolate into an even layer with an offset spatula. Cool completely, then refrigerate for about 30 minutes until the chocolate topping is firm.
Lift the brownie from the pan using the foil overhang and place it chocolate-side down on lightly greased parchment. Trace a 9-inch circle and cut it out for the cheesecake base. Place the circle, chocolate-side up, into the bottom of a 9-inch springform pan. From the remaining brownie, cut two triangular eyes, a smaller triangular nose, and a wide half-moon mouth. Cut small notches from the mouth to create teeth.
Sprinkle the gelatin over ¼ cup water in a small microwave-safe bowl and let it bloom for about 5 minutes. Meanwhile, beat the cream cheese, pumpkin puree, powdered sugar, sour cream, vanilla extract, and salt until smooth. Microwave the bloomed gelatin for about 20–30 seconds, just until melted, then stir it into the pumpkin mixture until evenly combined.
Pour the pumpkin filling over the brownie base in the springform pan and smooth the top. Gently press the brownie eyes, nose, and mouth into the filling, positioning them to create a jack-o'-lantern face. Keep the chocolate-covered side facing up and wipe away any filling that gets on the decorations.
Refrigerate the cheesecake for at least 4 hours, preferably overnight, until completely set. Tint the vanilla frosting with green gel food coloring and pipe a small stem at the top of the pumpkin face. Use melted semisweet chocolate to add any finishing details, if desired. Release the springform pan carefully, slice with a warm, clean knife, and serve chilled.