Preheat the oven to 350°F (175°C) and line an 8×8-inch pan with parchment, leaving some overhang for easy lifting.
Whisk flour, baking powder, and salt in a bowl.
In another bowl, whisk melted butter and sugar until smooth, then add eggs one at a time and stir in vanilla.
Divide the mixture into two bowls: stir cocoa powder and chocolate chips into one, and pumpkin purée with pumpkin pie spice into the other.
Spread the chocolate batter evenly in the pan, then top with the pumpkin batter and smooth gently.
Bake for 25–30 minutes until a toothpick comes out with a few moist crumbs.
Cool completely, then pipe green frosting into vines and decorate with mini pumpkin candies for the pumpkin-patch effect.