Preheat oven to 350 degrees F (175 degrees C) and grease an 8x8 inch baking pan.
In a large bowl, whisk together melted butter, 1 cup sugar, and vanilla.
Beat in 2 eggs one at a time until fully incorporated.
Sift together flour, cocoa powder, salt, and baking powder in a separate bowl.
Fold the dry ingredients into the wet chocolate mixture until just combined.
In a separate medium bowl, beat softened cream cheese until smooth.
Mix pumpkin puree, 1/4 cup sugar, egg yolk, cinnamon, and nutmeg into the cream cheese until uniform.
Spread approximately 3/4 of the chocolate batter into the bottom of the prepared baking pan.
Dollop the pumpkin mixture evenly over the chocolate base.
Drop the remaining chocolate batter in small spoonfuls over the pumpkin layer.
Gently swirl the layers using a knife or skewer to create a marble effect.
Bake for 30 to 35 minutes or until a toothpick inserted into the center comes out with only a few moist crumbs.
Allow brownies to cool completely in the pan on a wire rack before slicing into 16 squares.