Preheat your oven to 350°F (175°C).

Grease and line a 9x13-inch baking pan with parchment paper.

In a large bowl, whisk together melted butter and sugar until glossy and combined.

Add eggs one at a time, beating well after each addition, then stir in vanilla extract.

In a separate bowl, whisk flour, cocoa powder, salt, and baking powder.

Gradually fold the dry mixture into the wet mixture, being careful not to overmix to maintain fudginess.

Pour half of the brownie batter into the prepared pan.

Drizzle half of the caramel sauce over the surface, then add the remaining batter and smooth the top.

Finish by swirling the rest of the caramel on top using a butter knife for a marbled effect.

Bake in the center of the oven for 28–32 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs.

Let cool completely in the pan on a wire rack.

Once brownies are cool, cut them into squares.

Place a large marshmallow on each brownie, securing it with a small dab of caramel if necessary.

Using a kitchen torch or by placing under a broiler for a few seconds, gently toast the marshmallows until puffed and lightly golden.

Stick two mini chocolate chips into each marshmallow to create 'eyes.'

Lightly dust with powdered sugar for a ghostly effect and drizzle extra caramel over each brownie.