Preheat the oven to 350°F (175°C). Line two 12-cup muffin pans with 24 cupcake liners.

In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt until evenly combined.

In a second bowl, whisk eggs, milk, oil, and vanilla. Pour the wet ingredients into the dry ingredients and stir just until combined.

Gradually stir in hot water until the batter is smooth and thin. Divide among the 24 liners, filling each about two-thirds full.

Bake for 18–22 minutes, rotating pans if necessary, until a toothpick inserted into the center of a cupcake comes out clean or with a few moist crumbs.

Cool the cupcakes in the pans for 5 minutes, then transfer to a wire rack to cool completely, approximately 45 minutes total.

Beat softened butter until smooth. Add powdered sugar in batches, then vanilla and 2 tablespoons milk. Beat until fluffy, adding remaining milk only if needed. Tint with green gel food coloring.

Arrange the cooled cupcakes close together in a 4-by-6 rectangle on a large food-safe serving board or tray. Spread green buttercream across the tops to create one connected grassy surface, gently bridging gaps between cupcakes.

Write RIP or simple crosses on 8 oval cookies using black decorating icing. Crush 10 chocolate sandwich cookies into fine crumbs, then sprinkle them in little mounds and paths over the frosting.

Press the decorated cookie tombstones gently into the frosted cupcake tops. Add gummy worms and candy pumpkins if using. Serve by letting guests pull away individual cupcakes from the display.