Preheat the oven to 350 degrees and line a small loaf pan with parchment paper.
Melt the butter and unsweetened chocolate together in a saucepan over low heat. Stir until smooth, then remove from heat.
Whisk in the sugar until combined. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Fold in the flour and salt until no streaks remain. Pour half the batter into the prepared pan.
Place the four Oreo cookies in a single layer on top of the batter. Cover with the remaining batter and smooth the surface.
Bake for 25 to 30 minutes until a toothpick comes out with a few moist crumbs. Cool completely in the pan.
Melt the white chocolate chips in the microwave in short bursts. Drizzle or pipe the melted chocolate over the brownies to form simple haunted house outlines and ghost shapes.