Preheat the oven to 350°F. Spray 5 narrow baking trays with nonstick spray.
Prepare brownie mix according to package directions. Fill each tray halfway with batter. Bake 15–18 minutes or until set and a toothpick comes out clean. Cool completely.
Beat the butter until smooth. Mix in the powdered sugar, cocoa powder, vanilla, and 2 tablespoons cream until fluffy. Add an extra tablespoon of cream if needed for spreading consistency.
Spread a thin layer of raspberry jam on each brownie tray.
Pipe or spread the frosting over the jam.
Decorate each tray with two edible Halloween toppers for a chilling “slash n stab” look.