Preheat oven to 350°F. Butter and line cake pans with parchment paper.

In a large mixing bowl, combine melted butter, sugar, flour, cocoa powder, Greek yogurt, vanilla, eggs, baking powder, baking soda, and salt. Mix until well blended.

Pour in the hot water and stir until smooth, scraping down the sides of the bowl to incorporate everything evenly.

Divide batter evenly between prepared pans and bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean. Cool cakes completely on a wire rack before frosting.

Meanwhile, prepare the fudge frosting. In a medium saucepan over medium heat, cook butter, sugar, cocoa powder, chocolate, and salt for 4–5 minutes, stirring until the mixture bubbles at the edges but does not boil.

Slowly pour in the cream, whisking constantly. Continue whisking until the mixture is smooth, glossy, and thickened. Remove from heat and stir in vanilla. Let cool for 2–4 hours, or until spreadable. For quicker cooling, set the saucepan in a larger bowl filled with ice and whisk until chilled.

To assemble, place one cake layer on a serving plate. Spread half the frosting evenly over the top, smoothing to the edges. Place the second cake layer on top and cover with the remaining frosting, swirling with the back of a spoon or butter knife. Leave the sides of the cake bare for a rustic look.

To make the marshmallow webs, microwave marshmallows in a microwave-safe bowl for 20–30 seconds until softened. Carefully pick up about 1/4 cup of the melted mixture with your fingers and stretch it between your hands to form long strands. Drape the strands across the cake to create a web effect, repeating until the cake looks spidery and festive.

Decorate with googly eyes, candy corn, Halloween sprinkles, and plastic spiders for a playful finish. Slice, serve, and enjoy!