In a medium mixing bowl, add white chocolate chips and matcha powder. If you are using white chocolate blocks, make sure to chop beforehand. This allows the chocolate to melt easily. Set aside.
In a small saucepan, heat the whipping cream and unsalted butter over medium-high heat, until bubbles start showing on the edge. The butter should be fully melted. This should take less than 30 seconds, so do not leave it unattended. Do not boil the whipping cream.
Immediately pour the whipping cream into the white chocolate chips. Use a spatula to mix until the white chocolate is melted and fully incorporated with no clumps.
Line the empty ice tray with plastic wrap. You can do it on a flat tray with parchment paper. Separate the melted chocolate into 8 equal portions. You can use two spoons to help. Place each ball slightly away from the other. Place in the freezer for at least 30 minutes.