Melt white chocolate wafers in the microwave at 50% power, stirring every 30 seconds, until smooth.
Spread a spoonful of melted chocolate into each mold cavity, coating the bottom and sides.
Spread Nutella on one cracker, top with another to make 6 sandwiches.
Press a sandwich into each mold and cover with more melted chocolate, tapping to release bubbles.
Freeze for 10 minutes until firm, then pop cookies out of the mold.
Pipe remaining melted chocolate over the tops in thin lines.
Top with sprinkles immediately and let set 5 minutes before serving.