Place a bowl under a strainer or a colander.
Put the strawberries inside the colander and mash with a potato masher.
Let the strawberries drain, otherwise your frosting will be watery. I drained them for close to an hour. Then I drank the drained strawberry juice. :)
In a medium bowl, beat the butter and cream cheese until smooth.
Add in the powdered sugar, gradually, and the mashed/drained strawberries.
Continue to mix on medium-low speed until the sugar and strawberries are well incorporated, and the frosting is smooth.
If the frosting is a bit runny because the strawberries didn't drain well, place the frosting in a plastic container, cover, and put it in the fridge for about 20 minutes.
The frosting makes 3 cups.