While the brownies cool, make buckeye topping. In a mixing bowl combine the softened butter and peanut butter and beat until combined, about 1 minute. Add the powdered sugar, salt and vanilla and stir until no clumps of powdered sugar remain. Spread the peanut butter mixture over the prepared brownies and use a spatula to form an even layer. Set aside.
In a microwave safe bowl, microwave the chocolate chips and heavy cream in 30 second intervals, stirring well between each until completely melted. Pour the chocolate ganache over the peanut butter layer and spread into an even layer. Place the brownies in the fridge and cool until the ganache is set, at least 1 hour.
Slice and serve! Keep well for up to 1 week in airtight container!