Preheat oven to 350 degrees Fahrenheit. Line a 9-by-13-inch baking pan with parchment paper, and set aside.
To make Brownie Layer:
In a large bowl, using an electric mixer, cream together the butter and sugars until smooth. Add the vanilla and eggs, and beat until smooth.
In a medium bowl, whisk together the flour, cocoa powder, and salt. Add to creamed mixture in thirds, beating well after each addition. Stir in the chocolate chips.
Spoon brownie batter into the prepared pan, and bake for 30 to 35 minutes, or until a toothpick inserted in the center comes out mostly clean. Set aside to cool completely.
To make Pumpkin Layer:
In a large bowl, using an electric mixer, cream together the butter, pumpkin, and vanilla until blended.
Add salt, nutmeg, cinnamon, and 1 cup powdered sugar, and beat until smooth. Continue adding powdered sugar, 1 cup at a time, until mixture reaches the consistency of a thick frosting. Add a few drops of food coloring, if desired, and mix well.
Spread pumpkin mixture over cooled brownie layer in pan.