Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease an 8x8 inch baking pan. Stir the flour, baking powder, and salt together in a bowl.

Step 2

In another bowl, stir together the melted butter, sugar, and vanilla extract; beat in the eggs one at a time with a spoon. Gradually add the flour mixture, and stir the batter until it's evenly moistened. Divide the batter in half in two separate bowls.

Step 3

Into one bowl of batter, blend the cocoa powder and chocolate chips. In the second bowl of batter, stir in the pumpkin puree, walnuts, cinnamon, cloves, and nutmeg.

Step 4

Spread 1/2 of the chocolate batter into the bottom of the prepared baking pan, and follow with 1/2 of the pumpkin batter. Repeat the layers, ending with a pumpkin layer, and drag a kitchen knife or small spatula gently through the layers in a swirling motion, to create a marbled appearance.

Step 5

Bake in the preheated oven until the brownies begin to pull away from the sides of the pan, and a toothpick inserted into the center comes out clean, 40 to 45 minutes. Cool in the pan, cut into squares, and serve.