Preheat oven to 375 degrees.
Place the butter and water in a saucepan. Bring to a boil, remove from heat and whisk in the cocoa powder. Sift together the sugar, quinoa flour, baking powder, baking soda and salt. Add the cooled cocoa mixture, egg yolks, vanilla and sour cream and blend well. Beat the egg whites until stiff but not dry. Fold into batter.
Spoon into a muffin tin lined with paper cupcake liners. Bake for 20 minutes, or until a cake tester inserted in center comes out clean. Allow the cupcakes to cool to room temperature.
For the frosting, use a hand mixer to beat the butter and Nutella on high until evenly combined. Add the powdered sugar a quarter cup at a time, beating between each addition until all of the powdered sugar is added. Beat on high until frosting is smooth. (Add a splash of milk if needed to get the frosting to an easy spreading consistency.) When the cupcakes are cool, liberally spread icing over the top of the cupcakes.
Serve and enjoy!