Prepare cake mix as directed on package, adding vanilla. Stir in 1/4 teaspoon of the black food color to tint cake batter gray. Spoon batter into 24 paper-lined muffin cups.
Bake as directed on package for cupcakes. Cool cupcakes on wire rack.
Meanwhile, tint 1 container of frosting black with remaining 1 teaspoon black food color. Frost cupcakes, reserving 1/4 cup of frosting. Spoon reserved frosting into small resealable plastic bag. Snip a small corner from bag. Pipe “RIP” onto top half of each cookie. Push tombstone cookies gently into cupcakes.
For the ghosts, spoon remaining container of frosting into large resealable plastic bag. Snip a corner from bag. For the body of the ghost, pipe 1/2-inch dollop next to tombstone . For the head of the ghost, pipe a smaller dollop on top of the body, lifting while squeezing bag so that top of dollop is pointed. Use chocolate sprinkles and chocolate chips to decorate the ghost’s face.