Preheat the oven to 350°F and line a muffin tin with cupcake liners.
Prepare the cake mix according to package instructions, replacing the water with pineapple juice at a 1:1 ratio.
Bake according to package directions. Let cupcakes cool completely before frosting.
In a large bowl, beat the softened butter for 1 minute until smooth and creamy using a hand mixer or stand mixer.
Add the marshmallow fluff and beat until fully combined.
Add half of the confectioners’ sugar and beat on low speed until incorporated.
Add in remaining ¼ cup pineapple juice and the vanilla extract, then beat until smooth.
Add the remaining confectioners’ sugar and beat until light and fluffy, about 2 to 3 minutes.
Adjust frosting consistency as needed (see notes!).
Transfer frosting to a piping bag and pipe onto cooled cupcakes.
Garnish with fresh pineapple or desired toppings, if using.