Preheat oven to 350 degrees. Line two 12-cup cupcake pans with cupcake liners.
CAKE: Prepare Devil's Food cake mix according to package instructions. Fill each cupcake liner about two-thirds full with batter.
Bake until a toothpick inserted into each cupcake comes out clean, about 12-15 minutes. Let cool for 10 minutes.
Using a cupcake corer or teaspoon, scoop a small “well” out of each cupcake (reserving cupcake crumbs for garnish). Place a marshmallow into each “well” and bake until the marshmallow puffs, about 3-5 minutes. Let cool to room temperature.
FROSTING: In a large bowl using a hand mixer, beat butter, confectioners’ sugar, cocoa powder, vanilla, and salt. Beat in heavy cream (adding more by the tablespoon until consistency is creamy but can hold peaks).
Use a piping bag with a 1M tip to pipe frosting onto cupcakes. Sprinkle cupcakes with reserved crumbs and serve.