Making the Chocolate Cupcakes:

1. Preheat your oven to 350°F (about 175°C). Line a 12‑cup muffin tin with cupcake liners.
2. In a stand mixer bowl, combine softened butter, sugar, cocoa powder, and boiling water. Beat on low until sugar is dissolved and mixture is smooth.
3. In another bowl, whisk together flour, salt, baking powder, and baking soda. Set aside.
4. In a third bowl, beat the egg and then mix in sour cream and vanilla extract until smooth. Set aside.
5. With the mixer on low speed, add half the dry ingredients to the butter/sugar/cocoa mixture. Then add the sour cream/egg/vanilla mix. Finally, add the rest of the dry ingredients. Mix just until flour is incorporated—overmixing will reduce moisture and fluff.
6. Fill each cupcake liner roughly 2/3 full with batter.
7. Bake in the preheated oven for about 20–25 minutes, or until a toothpick inserted into the centre of a cupcake comes out clean.
8. After baking, let the cupcakes cool in the pan for about 5 minutes, then remove and allow to cool completely before frosting.

Making the Marshmallow Buttercream Frosting:

1. In the bowl of your stand mixer, beat the softened butter and marshmallow fluff until creamy and smooth.
2. Add vanilla extract and heavy cream; beat until well incorporated.
3. On low speed, gradually add confectioners’ sugar. Once all is incorporated, raise speed and beat for about 1 minute until light and fluffy. If frosting seems too thin, add more powdered sugar; if too thick, add a bit more heavy cream. If very sweet, a pinch of salt helps balance.

Assembling the Ghosts:

1. Using a piping tip (a Wilton #12 round tip or similar works well), pipe a swirl or mound of frosting onto each cooled cupcake to form your “ghost” body.
2. Immediately press two eye candies or mini chocolate chips into each ghost. If you want a “boo” mouth or smaller ghost‑faces, you can add small dots or shapes.
3. Serve immediately, or store as directed below.