Add the 3 cups of strawberries to a food processor or blender. Pulse until the strawberries are pureed. You should have about 1 ½ cups or just a little more strawberry puree.
Add the puree into a medium saucepan over medium heat and let it cook down to roughly ½ cup of puree. Mine actually measured out to be ½ cup plus ½ tablespoon. If you have more than that, pop it back into the saucepan and continue to cook it until it's reduced ½ cup. The time may vary, but it took 22 - 24 minutes for me. Remove from the heat and let cool to room temperature. *see notes
Add the butter, vanilla extract, and 4 cups of powdered sugar to a large mixing bowl. Using a hand mixer, beat on low until most of the powdered sugar is incorporated.
Add in half of the strawberry puree and continue to beat until incorporated.
Beat in the remaining powdered sugar into the bowl. Scrape down the sides of the bowl.
Add the remaining strawberry puree to the frosting and beat until incorporated.
Spread the frosting on top of cupcakes, cakes, etc. It can be piped into a shell border or rose swirl using the Wilton 1M tip.
*If you're finding the puree is sticking to the sides of the pan, push some of the puree from the bottom of the pan up on the sides where the stuck on puree is to moisten it up and then quickly scrape it off using a silicone spatula. Stir it back into the puree in the bottom of the pan and continue to cook. Do this as often as you need to to help keep the puree off the sides of the pan.