In a bowl, whisk together the egg yolks, salt, cornstarch, vanilla extract, and half of the sugar until smooth.
Heat the milk, matcha powder, and the rest of the sugar in a saucepan until simmering.
Pour the hot milk into the egg yolk mixture while whisking vigorously. Pour everything back into the saucepan and cook it over medium heat, whisking constantly until it thickens and becomes pudding-like in consistency.
Remove the pan from the heat and stir in the butter until it’s melted.
Transfer the mixture to a bowl and cover it with plastic wrap, pressing it directly onto the surface of the custard to prevent a skin from forming.
Chill the custard in the fridge until cold or use the freezer to speed up the process.
In a large bowl, whip the heavy cream until medium peaks form.
Add the chilled custard ⅓ at a time and whip into the whipped cream until well combined and stable.