Preheat oven to 350°F (175°C). In a large mixing bowl, combine cake mix with required ingredients per package instructions. Grease baking pan with cooking spray, pour batter in, and bake for 25-30 minutes.

Remove cake and cool in the pan for 10 minutes, then transfer to wire rack. Level the top if rounded.

Once cooled, generously dollop buttercream frosting on top and spread evenly.

In a microwave-safe bowl, heat mini marshmallows in 30 seconds until gooey, stirring frequently.

Drizzle melted marshmallows over the frosted cake in a spiral pattern to create a cobweb.

Knead black fondant until soft, then shape into a spider and place it on top of the cobweb.

Let the cake sit for a few minutes to allow marshmallows to set, then serve.