If wanting to make an iced pistachio matcha latte, add pistachio butter directly to the warmed milk and mix or froth until well combined. Fill a large glass or tumbler with ice and add the pistachio milk. Pour the hot matcha on top, stir, and enjoy.
I use Pistakio brand pistachio spread. It's only five ingredients with no artificial preservatives, has strong pistachio flavor, and is affordable compared to other pistachio butters. Your flavor may differ if using a different brand.