Line a 9-inch springform pan with plastic wrap so the edges overhang, then 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 the cake, remove the plastic wrap, and coat the top and sides with the whipped cream. Mix the crumbs with the melted butter and press the mixture onto the sides.
Scatter the remaining sprinkles over the top and add birthday candles just before serving.