1. Combine white chocolate and cream in a heatproof bowl set over a saucepan of barely simmering water. Heat, stirring occasionally with a rubber spatula, until chocolate is almost completely melted. Remove from heat and continue to stir until remaining chocolate is melted and mixture is thoroughly combined.

2. In a small bowl, combine matcha with cold water, adding one teaspoon at a time to reach same consistency as white chocolate ganache. Add green tea mixture to ganache; stir until incorporated. Cool to room temperature, stirring occasionally. If ganache remains too loose to pipe, briefly stir over an ice bath until thick enough to pipe, being careful not to let it become too firm.