Place 1 1/2 cups worth of the berries in the blender or food processor and puree until smooth, stop to scrape the sides as needed. This should only take a minute or two. Place the butter in a large mixing bowl and beat until smooth and creamy, add 1 cup of powdered sugar and beat again. Add 1 more cup of powdered sugar and beat to combine.
SLOWLY add just 1/2 cup of strawberry puree (you will NOT use all of the berry puree) and beat again until smooth. Add the remaining powdered sugar and vanilla and beat until smooth and fluffy.
Add more strawberry puree ONLY as desired, for a creamier or pourable icing (the softer icing will only work for a sheet cake, it will need to be as described above for a layered cake) or add additional powdered sugar, for a thicker and more pipe-able frosting for decorating.
Use the remaining berries to decorate the cake as desired. Store the decorated cake in the refrigerator until ready to serve. Enjoy!
PLEASE NOTE: The directions for this recipe will make more berry puree than you will need in order to make this frosting. I haven't found any way around this, simply because it requires a minimum amount of berries in order for the blender to puree them. Do NOT dump all of the puree into the butter/sugar mixture or you will have a soupy mixture on your hands. I save the leftover berry puree and add it to smoothies or stir it into oatmeal. You can also freeze the pureed berries in an ice cube tray and toss the cubes in a ziploc bag in the freezer until you are ready to use them.