Using an electric mixer, cream together butter and cream cheese until smooth.
Add vanilla extract and powdered sugar 1 cup at a time, mixing after each addition.
Using a spatula or large spoon, stir in diced strawberries.
A reader made a great observation that dicing the strawberries could lead to too much liquid coming out of the berries. If your strawberries are particularly ripe or juicy, consider slicing them or chopping in larger pieces to minimize the amount of liquid that they produce. Another idea is to simply not mix the berries into the frosting but instead make the frosting without the strawberries and then sprinkle the berries on top of the frosting once it's been added to the cake.