Preheat oven to 350, and line an 8x8 baking dish with parchment paper.
Mix together melted butter, sugars, and vanilla until well beaten. Add the eggs one at a time, beating well between each addition. Add the baking powder and salt and mix well again. Add the flour and mix until just combined.
3/4 cup unsalted butter, 1 cup brown sugar, 2 teaspoons vanilla, 3 eggs, 1/2 teaspoon baking powder, 1/2 teaspoon salt, 3/4 cup flour, 1/2 cup granulated sugar
Separate the batter into two bowls, one that has about 2/3 of the batter (bowl 1) and the other with 1/3 (bowl 1). Add the cinnamon, nutmeg, and pumpkin puree to the bowl with 2/3 of the batter and mix until just combined. Take some batter from bowl 1 and add it to bowl 2 until the batter is evenly split between the bowls.
In bowl 2, add the cocoa and half the chocolate chips and stir until just combined.
1/4 cup cocoa powder, 3/4 cup chocolate chips
Place half the chocolate batter into greased pan. Add spoonfuls of pumpkin mixture atop it. Spoon remaining chocolate atop that. Use a spatula carefully swirl it up just a little bit. Sprinkle chocolate chips on top.
Bake at 350 for 45 minutes - until tester comes out clean. Let it cool fully in the pan, then cut into squares and serve.
Store in a tightly sealed container on the counter for 2-3 days or freeze up to a month.