Preheat oven to 350 degrees Fahrenheit.
Prepare the cake, according to box directions.
Pour the cake batter into a parchment lined jelly roll pan.
Bake for 18-20 minutes, until top is golden and the cake is baked through in the center.
Allow the cake to cool.
Meanwhile, make the frosting. In a mixing bowl, beat the butter on medium speed with a hand or stand mixer.
Add about half of the powdered sugar, and mix to combine.
Add the salt, vanilla, and a splash of the milk. Mix to combine.
Add the remaining powdered sugar, and mix until everything is well combined. If needed, add another splash of milk and mix to combine.
Add the prepared frosting to a piping bag with a large round tip.
To assemble the cakes, use one of the cups to cut a circle out of the cake.
Add the circle to the bottom of another cup, pressing the cake down into the cup.
Add a swirl of frosting.
Cut out another circle cake piece and add it to the cup.
Top with another swirl of frosting and smooth it out evenly.
Press the top frosting into the sprinkles. Repeat for all of the cups.