Spread a thin layer of chocolate buttercream frosting over the top of each cooled cupcake using a knife or offset spatula.

Place the Oreo cookies in a zip-top bag and crush them with a rolling pin until they form fine crumbs. Sprinkle the crumbs evenly over the frosting on each cupcake to create the dirt layer.

Use the white icing to pipe simple "RIP" letters onto one side of each rectangular chocolate cookie. Let the icing set for a minute.

Press one decorated rectangular cookie upright into the Oreo crumbs on each cupcake to act as a tombstone. Lay one gummy worm on top of the crumbs near the base of the cookie.