Cook the berries over low in a small saucepan until warm and juicy. Press the mixture through a fine mesh sieve. Cool completely.
Frosting
In the bowl of a stand mixer, cream the butter until smooth.
Add in the powdered sugar a cup at a time, alternating with the berry puree.
Mix on low speed for 30 seconds and then medium speed for about a minute. Stop the machine, scrape down the sides and mix again. If the frosting appears too lumpy and dry, add a tablespoon of milk, only if needed. Mix again.