To make frosting, beat butter with electric mixer on medium speed 3 minutes. With mixer on low speed, beat in 3 cups of the powdered sugar until sugar is incorporated with butter. With mixer on medium speed, add salt, vanilla and 2 tablespoons of the milk; beat 3 minutes. If frosting needs more stiff consistency, beat in up to 1 cup powdered sugar. If frosting needs to be thinned out, add remaining milk, 1 tablespoon at a time. Frost cupcakes with frosting. Garnish with strawberry fans and blueberries. Store covered in refrigerator.