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Preheat oven to 350 F. Line a muffin pan with cupcake liners or place self-standing cupcake liners on a baking sheet.
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In a large bowl, combine sugar, flour, baking soda, baking powder, salt, and cocoa powder.
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Add egg, milk, oil, and vanilla extract and beat on medium speed until fully combined, about 2 minutes.
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Stir in boiling water (batter will be thin and runny).
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Pour batter into prepared cupcake liners and bake for 15 minutes or until a toothpick inserted into the center of the cupcake comes out clean.
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Cool cupcakes in the pan for 10 minutes before transferring to a wire rack to cool completely.
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Once cupcakes have cooled, use an apple corer to remove the center of each cupcake.
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Transfer marshmallow fluff to a piping bag fitted with a medium round piping tip.
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Pipe marshmallow fluff into the center of each cupcake. Set aside until ready to assemble.