Steps
- Preheat the oven to 350°F / 175°C. Line two baking sheets with parchment.
- Beat ½ cup of softened unsalted butter, 1 cup of packed light brown sugar, 1 teaspoon of baking powder, ½ teaspoon of baking soda, ¾ teaspoon of salt, and 1 teaspoon of vanilla extract until smooth. Beat in 1 large egg.
- Stir in ½ cup of Dutch-process cocoa. Beat in 2⅓ cups of all-purpose flour alternately with 1 cup of milk until smooth, scraping the bowl.
- Scoop ¼-cup mounds onto the sheets, leaving room to spread. Bake for 15-16 minutes, until firm. Loosen while lukewarm, then cool completely.
- Beat 1 cup of vegetable shortening, 1 cup of icing sugar, and 1⅓ cups of marshmallow spread together. Dissolve ¼ teaspoon of salt in 1 tablespoon of water, then beat in with 1½ teaspoons of vanilla extract.
- Spread the filling over the flat sides of half the cakes. Sandwich with the remaining cakes.