Preheat oven to 180 degrees C. Grease a 20cm square baking pan. Stir the flour, baking powder and salt together in a bowl.
In another bowl, stir together the melted butter, sugar and vanilla essence; 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.
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.
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.
Bake in the preheated oven until the brownies begin to pull away from the sides of the pan, and a toothpick inserted into the centre comes out clean, 40 to 45 minutes. Cool in the pan, cut into squares, and serve.