Preheat oven to 350°F. Line a cupcake pan with 6 liners.
In a large bowl or stand mixer, cream butter with sugar. Mix in egg and vanilla, scraping bowl as needed. Add remaining dry cupcake ingredients, mixing until fully incorporated. Finally, slowly add in milk, mixing just until no streaks remain and batter is smooth. Using a large cookie scoop, add one scoop to each liner. Bake for 18-20 minutes or until a toothpick inserted comes out clean. Cool in pan for 5 minutes, then transfer to a wire rack to cool completely.
In a heatproof bowl over a saucepan of simmering water, whisk together egg white, sugar and cream of tartar, until sugar dissolves. Continue whisking over simmering water until egg white mixture is warm to the touch. Remove from heat. Using a hand mixer or stand mixer, beat until still peaks form. Add in vanilla in last 30 seconds.
Transfer marshmallow to a piping bag with round piping tip or a large plastic bag with the corner cut off. Hollow out the centers of each cupcake, using a sharp knife. Be careful not to cut all the way to the bottom. Fill cupcakes with marshmallow.
In a large bowl or stand mixer, cream 4 Tbsp of butter. Sift together powdered sugar and cocoa powder, mix together with butter. Mix in vanilla and enough milk to desired piping or spreading consistency. Frost cupcakes immediately.
Nutritional information is provided as a courtesy and is an estimate only. Nutrition information can vary for a variety of reasons. For the most accurate data, use your preferred nutrition calculator with the actual ingredients you used to make this recipe.
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