Preheat oven to 350 degrees F (175 dgrees C). Make sure your oven is set to fit two cupcake pans. Line two 12-cupcake pans with paper cupcake liners.
In a medium bowl, mix together Cake Flour (1 3/4 cup), Unsweetened Cocoa Powder (1 cup), Baking Soda (1 1/4 teaspoon), and Salt (1/4 teaspoon) with a whisk. Set aside.
In a bowl of an electric mixer, beat Unsalted Butter (1 1/2 cup) on medium-high speed with a paddle attachment until creamy, about 30 seconds. Gradually add Granulated Sugar (1 1/2 cup) and beat at high speed until light, about 3 minutes. Reduce the speed to medium and add the Egg (2), one at a time, mixing well after each addition and scraping down the sides of the bowl as necessary. Mix in the Vanilla Extract (2 teaspoon).
Reduce the speed to low and begin adding Sour Cream (1 3/4 cup) and dry ingredients, alternating between the two and mixing between additions. Mix until just until blended.
Divide the batter evenly among the cupcake cups. Bake for 20 minutes, until a toothpick inserted into center of a cupcake comes out clean.
Place a large metal bowl and whisk in freezer now for about 15 minutes so they are good and cold for the whipped frosting. Cool cupcakes for at least 5 minutes.
Using a teaspoon, scoop a small well out of each cupcake. Place a marshmallow into each well and return to the oven to bake until the marshmallow puffs, about 5 more minutes. Remove the cupcakes and let them cool to room temperature.
Remove the mixing bowl from the freezer and pour Heavy Cream (1 cup) into bowl. Beat on high for 2 minutes until fluffy, then add Hot Chocolate Mix (1 cup) and continue to beat until stiff peaks form. Keep refrigerated.
Use a piping bag to frost cupcakes. Top with Marshmallow (24) if desired. I even used our mini kitchen blow torch to lightly toast the marshmallows.