Preheat oven to 350 degrees F. Sift together flours, cocoa powder, baking soda, baking powder and salt. Whisk to combine. In a large bowl or stand mixer, beat eggs and sugar until they are pale yellow and thick in consistency. Add in butter and mix to combine. Add Nutella and buttermilk and beat until incorporated.
Add in flour 1-2 cups at a time, mixing in after each addition. Scoop batter to fill lined cupcake tins 2/3 full. Bake for 22-25 minutes until tops bounce back when touched lightly. Allow to cool 5-10 minutes.
When cupcakes are still warm, fill a pastry bag with a round tip with the 1/2 cup Nutella. Insert tip into the top of each cupcake in the center about 1/2″ down. Squeeze in Nutella, about 1 tablespoon for each cupcake. Allow cupcakes to cool completely.
Beat together butter and Nutella until creamy. Mix in vanilla extract. Add in powdered sugar and mix well. Add cream until desired consistency is reached.