Preheat oven to 350 degrees. Line a standard muffin pan with cupcake liners, set aside.
First warm the ½ cup of water in the microwave and then add the espresso powder to it and give it a quick stir so the espresso powder dissolves. The liquid should resemble a cup of coffee. Set liquid a side for just a moment. In a large bowl combine cake mix, instant pudding mix, sour cream, oil & eggs and mix together using a handheld mixer (or stand mixer) on medium speed just until combined. Scrape down the sides of the bowl as necessary. Add in the water with the espresso, vanilla extract and a pinch of salt. Continue mixing on low speed just until the cake batter forms and is thoroughly combined. Do not over mix the batter - cake batter will be thick.
Fill the cupcake liners about ⅔ of the way full with cake batter. Bake cupcakes for 18 to 20 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool completely. Once cupcakes are room temperature use a cupcake corer or small knife to cut and punch out a small well in the center of the cupcake.
Make the marshmallow filling: in a medium bowl beat butter on medium speed for 2-3 minutes until fluffy. Add in powdered sugar and milk and beat on low speed until combined. Add in marshmallow fluff and a pinch of salt and beat mixture on medium high speed for 1-2 minutes until the filling is fluffy.
To fill the cupcake center with the filling you can either just spoon small amounts of filling into the center or fill a piping bag and cut off the tip and fill the cupcakes that way.
Make the chocolate icing: Add chocolate chips into a heat safe mixing bowl. In a small saucepan on low heat combine heavy cream, butter, corn syrup and espresso powder. Stir the ingredients and allow the mixture to come to a slight boil and then remove from heat. pour the mixture over the chocolate chips and let that set for about 2 to 3 minutes. Stir the chocolate chips and hot liquid together, the chocolate chips will melt and turn into a thin, glossy icing. Allow the icing to set at room temperature for about 20 to 25 minutes to thicken up.
When the icing is ready just take your cupcakes and simply turn them upside down and lightly dip the tops of the cupcake into the icing, allow any excess to drip off. Place cupcakes on a wire rack and let the icing set completely, icing will set on top of the cupcakes in about 5 minutes. Fill a piping bag with the remaining marshmallow filling and pipe the classic curlicue pattern on top. Cupcakes are ready to serve