Step 1: Puree the strawberries in a blender or food processor until smooth, then strain through a fine mesh sieve to remove seeds.
Step 2: In a large mixing bowl, beat the softened butter with an electric mixer on medium speed until creamy, about 2 minutes.
Step 3: Add the marshmallow cream and beat until fully combined and smooth.
Step 4: Gradually add the powdered sugar, one cup at a time, beating on low speed after each addition until incorporated.
Step 5: Mix in the strawberry puree, vanilla extract, and salt, then beat on medium-high speed for 3-4 minutes until light and fluffy.
Step 6: Use immediately to frost cakes or cupcakes, or refrigerate in an airtight container for up to 3 days.