Preheat your oven to 325°F (165°C). Spray a 9-inch square baking pan with nonstick spray and line the bottom with parchment paper. Set aside.
In a small saucepan, melt the unsalted butter and 1 cup of chocolate chips over low heat, stirring constantly until the mixture is smooth. Remove from heat and set aside to cool slightly.
In a medium mixing bowl, whisk together the granulated sugar, brown sugar, and eggs until well combined and smooth.
In another bowl, combine the flour, baking powder, and salt. Slowly fold the dry ingredients into the wet mixture, ensuring there are no lumps.
Divide the mixture into two separate bowls. In one bowl, add the melted chocolate mixture and stir until smooth. In the other bowl, mix in the pumpkin puree, vegetable oil, cinnamon, and pumpkin pie spice. Stir in the remaining ½ cup of chocolate chips into the pumpkin batter.
Pour the chocolate batter into the prepared baking pan and smooth it into an even layer. Then, pour the pumpkin batter on top. Using a knife, gently swirl the two batters together to create a marbled effect.
Bake the brownies in the preheated oven for about 40 minutes, or until a toothpick inserted in the center comes out clean. Let the brownies cool completely in the pan before slicing.