Beat cake mix and crushed pineapple until very few lumps remain. Fill paper-lined muffin cups two-thirds full. Bake according to package instructions.
Cool completely on a wire rack before frosting.
To make the vanilla bean buttercream:
Beat butter until light and fluffy. Beat in vanilla bean paste and salt. Gradually beat in powdered sugar and milk until it reaches your desired consistency.
To make the flower cupcakes:
Use kitchen scissors to cut marshmallow into 5 horizontal "slices". Dip the sticky side of marshmallow into sprinkles. Position the marshmallows around the cupcake and top with a round candy.