Scoop spoonfuls of ice cream all over the top of the cooled brownies. Smooth into an even layer and place in freezer until the ice cream is frozen solid.
Make the fudge sauce - In a small microwave-safe bowl, melt the chocolate in 30 second increments, stirring in between, until melted and smooth. (Careful not to scorch or overheat it!)
Remove from microwave and stir in the cocoa powder until smooth (it may be quite thick). Set aside.
In a medium saucepan over medium-low heat, combine the sugar, corn syrup, cream, salt and water. Stir constantly until the sugar is dissolved. Increase to medium-high and simmer, stirring frequently, for 3-4 minutes. Remove from heat and stir in vanilla and butter. Stir well to combine. Allow to cool for a few minutes, and then stir in the melted chocolate mixture until smooth. Allow mixture to cool completely.
Drizzle the cooled fudge sauce over the top of the ice cream and gently smooth it into an even layer. It's important that the sauce isn't warm or it will melt the ice cream as you are trying to smooth it on top. Return to the freezer for a few hours before serving. Slice into bars and, if desired, top with whipped cream and a maraschino cherry.