In the bowl of a stand mixer or a large bowl, beat the brown sugar and butter on medium speed until smooth and creamy.
Add in the flour and vanilla extract and beat until fully combined.
Mixing on low speed, add the milk slowly until fully incorporated.
Fold in the ½ cup mini chocolate chips. If the frosting is overly soft, pop into the fridge until desired firmness achieved. Spread over the cooled brownies and then top with remaining ½ cup mini m&ms.