Cut batter pouch open at dotted line.
Squeeze batter into prepared pan and spread evenly.
Bake 33-37 minutes. Cool completely before frosting.
For Frosting: Melt the butter in a small saucepan over low heat. Add the cocoa, stir until mixture is smooth and thick. Remove from the heat and transfer cocoa mixture to a medium bowl. Let cool slightly.
Add milk and vanilla alternately with powdered sugar to bowl and beat with an electric mixer or whisk until you have the spreading consistency you desire.
Spread frosting evenly over cooled brownies and scatter sprinkles over top.