Preheat oven to 350° F. Line a 9 × 9-inch pan with parchment and lightly coat with nonstick spray.
In a stand mixer fitted with the whisk attachment (or using a hand mixer), beat 3 eggs, ⅔ cup sugar, ¼ cup brown sugar, and 1 teaspoon vanilla on medium-high speed until the sugars dissolve, 1-2 minutes. Set aside.
4 eggs, 1 cup sugar, ¼ cup brown sugar, 3 teaspoons vanilla extract
Cut the butter into tablespoon-sized pieces and finely chop the chocolate bars. Place both in a large microwave-safe bowl.
½ cup melted unsalted butter, 8 ounces semisweet chocolate bars
Microwave in 30-second increments, stirring between each, until melted and smooth. Be careful not to burn the chocolate. Stir until fully combined.
While the chocolate mixture is still warm, stir in the egg-sugar mixture until incorporated.
Add the flour, cornstarch, cocoa powder, and salt. Gently mix until just combined. Do not overmix.
⅔ cup all-purpose flour, 1 Tablespoon cornstarch, ¼ cup cocoa powder, ¼ teaspoon salt
Fold in the chocolate chips.
¾ cup semisweet chocolate chips
Spread half of the brownie batter into the prepared pan.
In a medium bowl, beat cream cheese, pumpkin, remaining ⅓ cup sugar, remaining 1 egg, 2 teaspoons vanilla extract, pumpkin pie spice, and cinnamon until smooth.
8 ounces cream cheese, ½ cup pumpkin purée, 1 teaspoon pumpkin pie spice, ½ teaspoon cinnamon
Spread the cheesecake batter evenly over the brownie layer. Drop spoonfuls of the remaining brownie batter on top, then swirl gently with a knife.
Bake 35-40 minutes, until set or until a toothpick inserted in the center comes out mostly clean with a few moist crumbs. Cool in the pan for at least 1 hour, then lift out and let cool completely before cutting.