Preheat your oven to 350°F. Prepare two 12-count cupcake tins by lining them with paper liners. This ensures the cupcakes don’t stick and helps with easy removal once they're baked.
In a large bowl, add 1½ cups all-purpose flour, ⅔ cup unsweetened cocoa powder, 1 teaspoon baking powder, ¾ teaspoon baking soda, and ¼ teaspoon kosher salt. Whisk until the mixture is fully combined and well-aerated.
In a separate bowl, combine 1 cup brown sugar, 1 cup buttermilk, ⅔ cup vegetable oil, 1 teaspoon pure vanilla extract, and 2 large eggs. Whisk these ingredients together until smooth and creamy.
Gradually add the dry ingredients to the wet ingredients. Mix with an electric hand mixer on low speed until just combined. Be careful not to overmix, as this can lead to dense cupcakes.
Pour the batter evenly into the lined cupcake wells, filling each about two-thirds full to allow for proper rising. This helps achieve that perfect dome shape.
Bake in the preheated oven for 18 to 20 minutes. To check for doneness, insert a toothpick into the center of a cupcake. It should come out mostly clean with a few moist crumbs attached.
Once baked, transfer the cupcakes to a wire rack to cool completely. This is crucial, as filling them while warm can lead to melting.
While the cupcakes are cooling, prepare the marshmallow filling. In a mixing bowl, beat 4 tablespoons unsalted butter and ⅓ cup powdered sugar together until fluffy. Add 5 ounces marshmallow creme and mix until well combined.
Once the cupcakes have cooled, use a knife to carefully cut out a small circle in the middle of each cupcake, saving the tops. Pipe the marshmallow filling into the center of each cupcake until it’s filled to the top.
Take the saved tops, cut about one-third off the bottom, and place them back on top of the filled cupcakes. This creates that signature Hostess look!
In a microwave-safe bowl, combine ½ cup heavy cream and 1 cup semisweet chocolate chips. Heat in the microwave for 30 to 45 seconds, stirring until melted and smooth. Let it sit for a few minutes to thicken.
Dip the top of each filled cupcake into the chocolate ganache, allowing any excess to drip off. Place them back on the cooling rack to let the ganache set properly.
For the frosting, beat together 4 tablespoons unsalted butter and ½ cup powdered sugar. If necessary, add 1 tablespoon milk to achieve a smooth consistency.
Transfer the frosting to a piping bag or a sealable plastic bag with the edge clipped off. Pipe decorative swirls over the top of each cupcake, creating a beautiful finishing touch.