Preheat oven to 350 F. Line a 12 cup muffin pan with cupcake liners. In a large bowl, whisk together flour, sugar, salt and baking powder. Add in eggs, milk, vegetable oil and vanilla extract and whisk until smooth.
Fold in diced strawberries. Evenly divide the cupcake batter between the prepared liners. Bake for 18-20 minutes. Let cool completely.
Beat softened butter with smashed strawberries, powdered sugar, vanilla extract and milk until creamy.
Spread or pipe frosting onto cupcakes.