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 1 c. mini chocolate chips. If frosting is overly soft, pop into the fridge until desired firmness achieved. Note: You’ll want to use a big enough piping tip so the chocolate chips won’t clog the opening. I find it helpful to pipe the frosting alone and then simply top with chocolate chips.