Preheat oven to 375F. Spray a 36-count mini muffin tin with non-stick cooking spray. Set aside.
In a medium bowl, whisk flour, salt, baking powder, and cocoa powder. Set aside.
In another medium bowl, combine butter, semi-sweet chocolate chips and dark chocolate wafer. Melt in the microwave over medium heat, stirring occasionally, until smooth. Set aside to cool slightly.
In a large bowl, beat sugar and eggs until light and fluffy. Beat in vanilla extract. Stir in melted chocolate. Slowly stir in flour mixture until just incorporated.
Spoon batter into prepared muffin tin, filling about 3/4 full.
Bake for 9-11 minutes, or until brownies are set. Remove from oven and allow to cool completely in muffin tin.
To make the nutella frosting, beat butter in a large bowl with an electric hand mixer or stand mixer with fitted paddle attachment until light and creamy, about 2-3 minutes. Add in powdered sugar, 1 cup at a time, until incorporated.
Add Nutella, heavy cream, and vanilla. Beat for about 3-4 minutes, or until light and creamy. Pipe or spoon frosting on top of cooled brownie bites.