For the confetti cups: Line each cup of a 12-cup muffin pan with a cupcake liner.
Process 15 of the vanilla sandwich cookies into crumbs in a food processor. Add the melted butter, then mix to combine. Press the crumbs into the cupcake liners. Set aside.
Crush the remaining 4 vanilla sandwich cookies into chunks. Mix the softened ice cream with the crushed vanilla cookies and sprinkles in a bowl. Spoon into the cupcake liners.
For the toppings: Press the rainbow sprinkles and chocolate shavings into the top of each cup. Freeze until firm, at least 2 hours.
Remove the confetti cups from the muffin pan (leaving on the cupcake liners) and place in a single layer in resealable bags or other airtight containers.
To serve, peel off the liners and place each one on a plate. Top with sweetened whipped cream if you'd like. You can also just serve them in the liners.