Preheat oven to 350ºF degrees, and line a muffin pan with cupcake liners. Add the hot chocolate mix to the hot water, and stir until fully dissolved; set aside.

In a large mixing bowl, whisk the flour, cocoa powder, baking soda, salt, and sugar together until well combined. Add the oil and yogurt and continue stirring with a whisk. Batter will be thick so you’ll have to add half of the hot chocolate liquid to stir more easily. Add the rest of the hot chocolate liquid, the egg, and vanilla, stirring until just combined.

Pour batter into cupcake liners, filling 2/3 of the way. Bake in the oven for 16-18 minutes, or until a toothpick inserted in the center comes out clean. Allow cupcakes to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.

In a small saucepan over medium heat, whisk flour, hot chocolate mix, and milk together, stirring constantly, until it thickens. It should look like thick pancake batter or pudding. Remove from heat and immediately pour hot chocolate mixture into a bowl to cool completely.**

After the hot chocolate mixture has come to room temperature, use a stand mixer fitted with the whisk attachment or a beater to cream the butter, sugar, cocoa powder, vanilla, and salt together on medium high speed until light and fluffy, about 3 minutes.

Add the completely cooled hot chocolate mixture, and beat it for about 7-10 minutes on high until it looks like whipped cream. The mixture will look curdled at first, but as you continue whipping it, it should come together. If you can still see streaks of butter, keep beating it until you can’t.***

Scoop marshmallow fluff into a piping bag, fitted with a round tip. Alternatively, you can use a gallon ziplock bag and cut the corner tip off.

Using a sharp knife, carve a hole into the center of each cupcake, about halfway deep. Remove cake piece and fill with marshmallow fluff. Pipe the buttercream onto each cupcake. Garnish with mini marshmallows.