In a mixing bowl, mix together melted butter and cocoa powders. Add sugar and vanilla, and mix until just combined. Mix in eggs, one at a time. Add flour, one cup at a time, and mix until no flour streaks remain - batter will be lumpy. Fold in fudge sauce and mix until distributed.
Spread batter in a well greased 9x13 pan. Bake at 350 for 20 minutes. Brownies will come out very dense and thick.
Frost cooled brownies, and refrigerate before serving.
Cake Batter Icing:
In a mixing bowl, using a paddle attachment, beat butter and butter vanilla emulsion until smooth. Add powdered sugar one cup at a time and beat until well combined. Add heavy whipping cream and beat until fluffy. Gently fold in sprinkles.