Line a 9-inch springform pan with plastic wrap, leaving overhang. Place one cake half in the bottom.
Spread half the softened ice cream over the cake, smooth it, sprinkle with 2/3 cup sprinkles, and freeze for 15 minutes.
Spread the remaining ice cream on top, add the second cake half, cover, and freeze for at least 2 hours, until firm.
Unmold, remove plastic wrap, and coat the top and sides with whipped cream. Mix crumbs with melted butter; press onto the sides.
Scatter the remaining sprinkles over the top and add birthday candles just before serving.