Heat the oven to 350°F (175°C) and line an 8-inch square baking pan with parchment paper.
Melt the butter and dark chocolate together. Allow the mixture to cool briefly, then whisk in both sugars, eggs, and vanilla.
Fold in flour, cocoa powder, and salt just until combined. Pour into the prepared pan and smooth the surface.
Bake for 22–28 minutes, or until the edges are set and the center remains slightly fudgy. Cool completely before cutting into squares.
Melt the white chocolate gently. Spoon a small mound onto each brownie and drag portions outward with the back of a teaspoon to create irregular ghost bodies and arms.
Let the white chocolate partially set, then pipe tiny dark chocolate eyes and mouths onto each ghost. Add tiny pink decorations if desired.