Preheat oven to 350F and line mini cupcake cups with mini cupcake liners. Set aside.
In medium-sized bowl, whisk together flour, baking powder and salt and set aside.
In KitchenAid or with electric hand mixer, cream together butter, sugar and Nutella.
Beat in eggs, one at a time and scrape down the sides of the bowl to ensure ingredients are all incorporated.
Stir in vanilla extract.
Add flour mixture and milk, alternating (add approximately ¼ of the flour mixture then approximately ¼ of the buttermilk, stirring until just combined after each addition. Once the last of the flour/buttermilk has been added, scrape down the sides and the bottom of the bowl with a spatula and stir again until ingredients are completely combined.
Divide batter into prepared mini-cupcake tin, filling each liner ¾ full (I use a 1 ½" cookie dough scoop for this, it makes the process very clean and easy).
Bake on 350F for 14 minutes (a toothpick inserted in center of cupcake should come out clean).
Allow cupcakes to cool completely before frosting.
Frosting
In KitchenAid (or with electric hand mixer), cream together butter and cream cheese.
Add Marshmallow fluff and beat until well-combined.
Stir in vanilla extract.
Gradually, add in sugar, stirring until completely combined (pause occasionally to scrape bottom and sides of bowl so all ingredients are evenly incorporated).
Once mini cupcakes have cooled completely, spread or pipe icing on top.
Sift/dust a small amount of cocoa powder over each cupcake, if desired.
Notes
*A good substitute for buttermilk (one that I use all the time) would be ½ cup regular full-fat milk with ½ Tbsp (1 ½ tsp) vinegar or lemon juice stirred in and allowed to sit for 5 minutes before using